A standard US penny can hold a surprisingly tall dome of water on its face, typically accommodating somewhere between 20 and 40 drops from an eyedropper before the liquid finally spills over the edge. The exact number depends on temperature, how clean the coin is, the size of your drops, and how carefully you place them. But the reason any coin can hold that much water at all comes down to the interplay between water’s surface tension, the physical properties of the penny’s surface, and the way the liquid’s edge grips the rim of the coin.
Why Water Piles Up Instead of Spreading Flat
Water molecules are strongly attracted to one another. Each molecule forms temporary bonds with its neighbors through the electrical pull between their hydrogen and oxygen atoms. Deep inside a drop, a water molecule gets tugged equally in all directions by surrounding molecules. But at the surface, molecules only have neighbors beside and below them, not above. That imbalance pulls the surface inward, creating a kind of elastic skin. This is surface tension, and water has an unusually high amount of it compared to most common liquids, around 72 millinewtons per meter at room temperature.
That strong surface tension is what lets water pile into a dome on the penny rather than just spreading into a thin film. As you add drops, the water bulges upward because the surface layer resists being stretched. The dome can grow remarkably tall relative to the penny’s diameter. You can actually watch the profile become more and more convex with each added drop until the surface tension can no longer contain the weight of water pressing outward, and the dome collapses over the edge.
How the Penny’s Surface Interacts With Water
Modern US pennies are zinc coins plated with a thin layer of copper, and the way water behaves on copper is more complicated than you might expect. A perfectly clean copper surface is quite friendly to water; in the language of surface science, it is hydrophilic, meaning water tends to spread across it rather than bead up. Research using controlled atmospheres has found that fresh copper surfaces in an inert environment show contact angles as low as 44 degrees, meaning the water flattens out significantly.
But the penny in your pocket is not pristine laboratory copper. The moment copper is exposed to air, it begins accumulating a thin layer of oils, oxides, and airborne hydrocarbons. This contamination dramatically changes how water sits on the surface. Studies on copper exposed to normal air found that contact angles jumped into the range of roughly 50 to 82 degrees, shifting the surface from clearly water-attracting to nearly water-repelling depending on how long the exposure lasted.1PubMed Central. Revisiting Wetting, Freezing, and Evaporation Mechanisms of Water on Copper In practical terms, this means a grimy, well-handled penny behaves differently from one fresh out of a roll. A dirtier coin, somewhat counterintuitively, often holds more drops because the water beads up more tightly instead of spreading thin and running off the edge.
This is why cleaning a penny with vinegar or another acid before the experiment can actually reduce the number of drops it holds. Stripping away the accumulated grime exposes a more hydrophilic copper surface, and the water spreads flatter instead of building into a tall dome. If you want the highest drop count, a slightly oxidized penny that has been sitting in a drawer for a while often outperforms a freshly scrubbed one.
The Edge Matters More Than You Think
The rim of the penny plays a crucial role in containing the water dome, and the physics at work there is called contact line pinning. The “contact line” is the boundary where the water, the air, and the solid surface all meet, which in this case runs around the perimeter of the penny. When that line encounters a sharp geometric feature like the raised edge of a coin, the water’s advancing front gets stuck there. It cannot easily climb over the ridge and flow down the other side.
Research on how surface topography controls this pinning behavior has shown that the critical factor is the slope of whatever obstacle the contact line encounters. For much of the parameter space studied, neither the height nor the width of the surface feature mattered as much as the steepness of its sidewalls.2PubMed Central. Contact line pinning by microfabricated patterns: effects of microscale topography A penny’s rim is essentially a tiny cliff face, and that steep sidewall is what keeps the water from simply rolling off. This is consistent with a theoretical model predicting that sharp edges impose a geometric barrier that the contact line must overcome before it can advance further.
This is also why the experiment works much better with a penny than with, say, a smooth flat disc of the same diameter. The raised rim, the portrait relief, and the lettering around the edge all create miniature barriers that help anchor the water. A perfectly smooth, rimless disc of copper would hold noticeably less water because there would be nothing to pin the contact line in place.
How Temperature Changes the Result
Surface tension is not constant. It weakens as temperature rises, because warmer molecules move faster and the cohesive forces between them have less grip. Classic research on the surface tension of water established that this relationship is smooth and essentially linear: as the temperature goes up by a given amount, the surface tension drops by a predictable amount.3PubMed. Surface tension and surface structure of water At room temperature, water’s surface tension is around 72 millinewtons per meter. Near boiling, it falls to roughly 59. Near freezing, it climbs to about 76.
For the penny experiment, this means the temperature of the water and the room will nudge your drop count. Cold water from the refrigerator will have slightly stronger surface tension and should hold a few more drops than water that has been sitting out on a warm day. The difference is modest for typical indoor temperature swings, maybe a couple of drops either way, but it is real and reproducible. If you are doing this as a classroom experiment and want consistent results, using water at the same temperature each time eliminates one variable.
What Soap Does and Why It Works So Fast
Adding even a tiny amount of dish soap to the water causes the dome to collapse almost instantly. Soap molecules are surfactants, meaning each molecule has one end that is attracted to water and one end that is repelled by it. When these molecules reach the water’s surface, they wedge themselves into the surface layer and break up the cohesive network of water molecules. This drastically lowers the surface tension, often cutting it by more than half.
The effect is so dramatic that it is one of the most common additions to the penny experiment in classrooms. Students first count how many drops of plain water the penny holds, then repeat the experiment with soapy water. The soapy water typically manages only a fraction of the drop count, sometimes fewer than ten drops, because the weakened surface can no longer support a tall dome. The water spreads thin and spills over the edge quickly.
What makes this demonstration effective is the speed at which surfactant molecules migrate to the surface. You do not need to stir or mix thoroughly. Even dipping a toothpick into dish soap and touching it to the water dome on the penny is enough to cause an immediate collapse, because the surfactant molecules rush to the air-water interface where they are most thermodynamically comfortable.
Why Drop Counts Vary So Much Between Trials
If you repeat the penny experiment several times, you will likely get different numbers each time, and that is not a flaw in your technique. Several factors contribute to the variability:
- Drop size: A standard eyedropper does not produce perfectly uniform drops. Squeezing harder or faster creates larger drops, and larger drops deliver more water per count, so you reach the spill point sooner with fewer of them.
- Drop placement: Dropping water onto the center of the penny lets the dome grow symmetrically, which maximizes volume. Drops placed near the edge are more likely to trigger an early spill because they load one side of the dome disproportionately.
- Drop height: Releasing a drop from higher up gives it more momentum when it hits the dome, which can rupture the surface and cause an early overflow. Holding the dropper close to the surface minimizes this impact.
- Coin orientation: Even a slight tilt favors one edge, and gravity will pull the dome toward the lower side. On a perfectly level surface, the water distributes more evenly and the dome can grow larger.
- Surface contamination: Fingerprints, dust, and residual soap from a previous trial all change the surface energy of the coin. Two pennies from the same roll can behave differently if one was handled more.
This variability is actually why the penny experiment is popular in science education. It gives students a reason to talk about experimental controls, repeated trials, and averaging. The “right” answer is not a single number but a range, and learning to work with that range is the point.
What Happens Inside the Dome
The water sitting on the penny is not as still as it looks. Even before evaporation becomes significant, internal circulation patterns develop within the droplet. Research on evaporating water droplets has found that temperature differences across the drop’s surface drive internal flows called Marangoni convection. Because the edges of a sessile drop (one sitting on a surface) are thinner and closer to the air, they tend to cool faster through evaporation than the thicker center. Since surface tension is higher in cooler regions, the surface layer gets pulled from the warmer center toward the cooler edges, dragging fluid along with it and setting up circulation loops.
In laboratory measurements, these internal velocities were found to reach around 10 millimeters per second in the center of the droplet, with mathematical models predicting even higher speeds deeper inside.4International Journal of Heat and Mass Transfer. Marangoni convection in an evaporating water droplet The convection initially appeared as paired vortices in the midsection of the drop and remained stable for only about 30 seconds before transitioning into a single swirling pattern throughout the droplet.4International Journal of Heat and Mass Transfer. Marangoni convection in an evaporating water droplet So even while the dome looks perfectly calm to the naked eye, there is a dynamic system of flow happening inside, driven by the uneven evaporation at the surface. This is part of why larger droplets can sometimes be less stable than you would expect: the internal flows create small pressure variations that can nudge the dome toward its tipping point.
Heads or Tails
A question that comes up in classrooms is whether the heads side or the tails side of a penny holds more water. The Lincoln Memorial design on the reverse of older pennies has a different relief pattern than Lincoln’s portrait on the obverse, and the Union Shield on newer pennies is different still. In principle, the side with more raised detail could offer more pinning sites for the contact line, potentially holding a slightly larger dome. In practice, the differences are small enough that they tend to get lost in the normal trial-to-trial variability from drop size and placement. But the question is not silly, because the surface topography genuinely does matter. A heavily worn penny with almost no relief would lose some of the micro-barriers that help pin the water, and it would likely hold slightly less than a mint-condition coin with crisp, sharp features.
Some teachers extend the experiment to other coins: nickels, dimes, and quarters. Bigger coins have more surface area and a longer rim perimeter, so they hold more total water. But the relationship is not simply proportional to area, because the dome height is limited by surface tension regardless of the coin’s diameter. A quarter does not hold four times as much water as a penny. The dome still reaches a maximum height determined by the balance between surface tension and gravity, and wider coins just spread that dome over a larger footprint. Comparing drop counts across different coins is a useful way to start thinking about how area and volume scale differently.
Foreign Coins and Different Metals
Pennies are copper-plated zinc, but coins from other countries are made of steel, nickel, aluminum, brass, and various alloys. Each metal has a different surface energy when exposed to air, and that changes how water interacts with it. A stainless steel coin, for instance, tends to develop a very thin and stable oxide layer that makes it moderately hydrophilic, while aluminum coins form an oxide that can be either quite water-attracting or somewhat neutral depending on the alloy. The copper oxide and hydrocarbon layer on a US penny, as discussed earlier, pushes the surface toward less hydrophilic behavior, which is part of why pennies work well for this experiment.1PubMed Central. Revisiting Wetting, Freezing, and Evaporation Mechanisms of Water on Copper
Coins also differ in edge geometry. Some have smooth edges, some have reeded (ridged) edges like a US dime or quarter, and some have unusual shapes like the heptagonal British 50-pence piece. Reeded edges introduce many tiny vertical ridges, each of which acts as a potential pinning site. Whether reeded edges help or hurt depends on their orientation relative to the advancing water. If the grooves run perpendicular to the contact line, they can act like a series of tiny walls that help contain the dome. If the geometry allows the water to wick along the grooves, it could encourage spillage instead. In practice, most reeded-edge coins still hold a respectable dome because the grooves are fine enough that surface tension bridges across them rather than following each channel individually.
Alcohol, Saltwater, and Other Liquids
Swapping out pure water for other liquids changes the outcome dramatically, and this is another popular classroom extension. Rubbing alcohol (isopropanol) has a surface tension of only about 21 millinewtons per meter, roughly a third of water’s value. On a penny, alcohol spreads almost flat and spills after just a handful of drops. Saltwater, on the other hand, has slightly higher surface tension than pure water because dissolved ions strengthen the cohesive forces at the surface. A saturated salt solution might hold a few more drops than tap water, though the difference is subtle.
Glycerin and honey are interesting edge cases. Both are viscous, and viscosity slows down the flow that leads to spillage, but viscosity and surface tension are different properties. Glycerin has a surface tension close to water’s but is much thicker, so it resists flowing over the edge even when the dome is quite large. The result is a dome that looks lopsided and unstable but stubbornly refuses to spill. Honey is similar but even more extreme in its resistance to flow. These liquids illustrate that the penny experiment is really testing two things at once: how strong the surface skin is and how easily the bulk liquid flows once that skin is breached.
Milk falls somewhere between water and alcohol in surface tension because it contains proteins and fats that act as mild surfactants. Whole milk will typically hold fewer drops than water but more than alcohol. Skim milk, with less fat, holds slightly more than whole milk. These comparisons make the experiment a useful way to explore how dissolved or suspended substances alter the liquid’s surface properties without needing any specialized equipment beyond a coin and an eyedropper.