Here is a set of instructions from eHow.com on rock skipping:
1. Select a rock that's round, flat and smooth.
2. Stand at the edge of a large, placid body of water.
3. Hold the rock horizontally - flat side down - with your index finger curling around one edge.
4. Aim the rock. Envision a convex arc a few inches above the water.
5. Throw the rock low and parallel to the water's surface. Throw sidearm so that your hand travels past your waist and the rock travels horizontally across the water.
6. Release the rock with a snap of the wrist to give it a horizontal spin. Your elbow will be next to your hip as the rock leaves your hand.
7. Count the number of times the rock skips.
Also, an interesting nerdly ninja info:
You can skip rocks on sand, but on sand, the stone will bounce twice between skips. On water, it only bounces once between skips. Why?
A high school student, Kirston Koths, came up with a solution in 1967: "It turns out that when a spinning stone comes into contact with a surface, the trailing edge usually strikes first. If the surface is hard, like packed sand, the stone tilts forward, the leading edge strikes again, and the stone takes off on the next skip.
When a revolving stone hits a fluid surface, it behaves quite differently. When its trailing edge strikes, the stone doesn't tip forward. Instead, it tilts backward, causing a small wave to build up underneath it. After planing on this wave for a short distance, the stone then takes off again without the short hop that it takes on a dry surface." http://www2.gi.alaska.edu/ScienceForum/ASF7/731.html
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