HOW COME? The spin on skipping stones

When we throw a stone towards a lake, why does it sometimes bounce across the surface instead of just sinking? asks a reader.

Letting the frogs out. Making the rabbit leap. Ducks and drakes. Whatever you call it, people have probably skipped stones (and shells) for as long as there were humans.

And anyone who's ever tried bouncing a rock across a pond soon realizes there are rules. The stone's shape and size matter, as does the throw. Toss a rock slowly, and at a large angle, and it will promptly sink.

Why a stone skips across water is surprisingly complicated, and scientists are still doing experiments to figure out exactly how it works. The best skipping stones are flattish, neither too heavy nor too light. Calm water works better than a wind-ruffled surface. And throwing technique is crucial.

French experiments with a rock-skipping robot showed that the mass of the rock, the angle at which it contacts the water, and its speed and spin all determine whether a stone will skip (and for how long). Think of a skipping stone as a skimming water skier, scientists suggest. The stone and the skier both feel an upward force from the water. And this force increases with the skimming object's speed.

So a stone with greater initial velocity will be more likely to skip across the surface. And be sure to put a spin on it: A spinning stone's forward motion is actually stabilized as it flies through the air. When it hits the water, the stone's spin keeps it balanced. So instead of its motion degrading into a crazy somersault, the stone neatly skips along the surface.

Opinions (and experiments) vary, but the ideal way to throw a stone is so that it lands at a 10- to 20-degree angle from the surface of the pond. (No need to measure; simply toss the stone nearly parallel to the water.) Add a good spin with the flick of your wrist -- and throw hard enough to make the stone speedy -- and your rock should skip at least a few times. (One researcher found that for a stone to skip five times before sinking, it must be spinning at five times a second.)

What finally ends a stone's pond-skimming? With each small bounce, a tossed rock loses energy, experiences frictional drag from the water, and is also tugged down by gravity. So after a number of (slowing) skips, the stone sinks below the surface.