How many of you have walked beside a lake or a pond, then bent down to pick up a pebble, have thrown it level against the water and eagerly counted the number of skips it does before sinking in ? If you have'nt done it, then you are out of this universe. If you have done it, then welcome to the stone skippers club comprising of 99% of the guys in the known universe.
In case you belong to the club and have been wondering how to improve your skip count, the answer is in this equation.
τ = (MR/ρS)-1/2/U
Simple ain't it ? Looks like its all in the speed and the angle of delivery. Ok for the detailed explanation look at The Physics Web