Running on water has long captured the human imagination, appearing in myths and superhero tales. While impossible for humans, understanding the science reveals why some animals can perform this seemingly miraculous act.
The Physics of Water Running
Running on water involves overcoming two primary physical challenges: supporting weight and generating forward motion. For very small or light entities, water’s surface tension plays a role. Surface tension arises from the cohesive forces between water molecules, which attract each other more strongly at the surface than within the bulk liquid, creating a thin, elastic-like “skin” on the water’s top layer. This force is strong enough to support lightweight objects, such as a paperclip, if carefully placed.
However, for heavier objects like animals or humans, surface tension alone is insufficient. The more significant principle at play is hydrodynamic force, also known as dynamic lift. This force is generated when an object moves rapidly through water, pushing water downwards and receiving an upward reaction force, similar to how a boat planes on the surface or an airplane flies. The faster the object moves and the more water it displaces, the greater the upward force. This involves the rapid transfer of momentum from the foot to the water, requiring substantial force applied over a very short time.
The Speed Needed for Humans
For a human to run on water, the speed needed is far beyond natural capabilities. Physicists calculate that a human would need to achieve speeds of approximately 30 meters per second (about 67 miles per hour) to generate enough hydrodynamic lift. This immense speed is required to make contact with enough water quickly during each stride to create sufficient upward force before the foot sinks.
The challenge is not just speed but also power. To achieve such a speed, a human would need to generate approximately 12 kilowatts of mechanical power. For comparison, a trained athlete can typically produce only about half a kilowatt, illustrating the vast difference in required power output. Human feet also have a small surface area, which limits the amount of water that can be pushed down with each step.
Animals That Walk on Water
Some animals have evolved specialized adaptations to run or walk on water. Small insects, such as water striders, exemplify the use of surface tension. Their light weight, small size, and specialized legs, which are long, slender, and covered in tiny hydrophobic (water-repelling) hairs, allow them to utilize the water’s surface film without breaking through it.
Larger animals, like the basilisk lizard, employ hydrodynamic force. Often called the “Jesus Christ lizard,” the basilisk rapidly slaps its hind feet onto the water’s surface. These lizards possess large feet with fringes of skin between their toes, which splay out upon impact, increasing the surface area and creating air pockets beneath their feet. This action generates a powerful downward thrust, pushing water away and producing enough upward force to keep the lizard afloat for short bursts as it propels itself forward. They must maintain high speed to continue this locomotion.
Why Humans Cannot Run on Water
Humans are unable to run on water primarily due to insufficient speed and power, and unsuitable anatomy. We cannot generate the 67 miles per hour of speed or the 12 kilowatts of power necessary to create enough hydrodynamic lift to counteract our body weight. Our feet lack the large surface area, specialized fringes, or hydrophobic properties seen in water-walking animals. Unlike the basilisk lizard’s rapid slapping motion and fringed toes, human feet would simply plunge through the water. Our relatively high mass compared to the small surface area of our feet and our limited muscular strength makes it impossible to overcome the force of gravity using water’s resistance at any achievable speed.