Has a joker ever tried to show you that instead of having the obvious 10 fingers, he actually has 11? This is done by bending down and counting fingers on one hand backwards from 10: "10, 9, 8, 7, 6...." The joker then says "... and 5 makes 11".
Counting on one's fingers seems a natural way to compute numbers, but the ancient Greeks and Romans didn't just count "on" their fingers. They counted with their fingers, and not to be quick and accurate with the finger symbols could be embarrassing.
Read more about ancient finger counting (technical term: dactylonomy):
Finger Numbers.
Also see Laura Gibbs's Blog on Roman Multiplication by Fingers.
Comments
It says here in this book, Sibiller tribesman of New Guinea have a system of counting. Fingers, wrist, forearm, elbow, bicep, collarbone, shoulder, ear, eye, nose. Interesting book.