In the study of animal locomotion on the surface layer of water, Denny's paradox refers to the apparent impossibility of surface-dwelling animals such as the water strider generating enough propulsive force to move. It is named after Mark Denny.If capillary waves are assumed to generate the momentum transfer to the water, the animal's legs must move faster than the phase speed of the waves, given by where is the acceleration due to gravity, is the strength of surface tension, and the density of water. For standard conditions, this works out to be about 0.23 m/s.