複雜系統之簡介(梁鈞泰) - 中研院物理研究所- Academia Sinica

2. Mechanism of locomotion
To move, one must push on something backward, something that carries
the momentum. It's the ground (earth) that we push when we walk, and
vortices in water when we swim. How about for water striders?
Long believed to be surface waves (capillary waves).
But surface wave speed = (4 g σ/ρ) ¼ = 23 cm/s for water.
A strider must beat its legs faster than this speed.
No problem for adults, but measurements show that infant striders
can’t beat that fast.
Denny's paradox.