能在水上“散步”的动物

我们常在武侠小说里看到一门功夫叫“轻功水上漂”，说的是人可以在水上快速通行、如履平地。现实中，如果让一个人以正常、甚至更慢的速度在水面上走过，恐怕就是“轻功大师”也不行。但是，世界上有一种小动物就可以做到。

Walking on water may seem miraculous, but for tiny aquatic snails（水生蜗牛）, it's an everyday activity. Now, scientists have puzzled out the snails' baffling method of propulsion.

"How the snails were dragging themselves across a surface that they could not even grip was absolutely perplexing to us," said lead author Eric Lauga, a professor of mechanical and aerospace engineering at the University of California, San Diego.

"Hanging on to the water's surface is not the issue for the snails. They are naturally buoyant, because they are so small," Lauga said.

Even so, the snails need traction to move across the slippery surface. Think of humans trying to walk on ice—they don't break through, but their feet can't get a grip.

By making small rippling motions with its foot, the snail creates traction for itself, Lauga and his colleagues found after studying videos of the snails. The researchers' observations are detailed this month in the journal Physics of Fluids（流体物理杂志）.

The snails' ability to move depends on water's tendency for its surface to resist disturbance. Water "wants" to stay flat, Lauga said.

When the snail ripples its foot, similar ripples are created on the water's surface. The ripples generate a downward force as the water flattens itself.

These ripples are just the right size for the snail to use to push itself along. "If the ripples were too small, the snail would slip, as on ice," Lauga said.

"If the ripples were too big, the snail could not 'grab' them."