Performance :

Hold two coins flat on top of each other between your thumb and forefinger, sideways to the audience. Say you will make an extra coin by rubbing them together.

Rub the coins quickly backwards and forwards against each other. It looks as if there are three coins, not two.

It is almost certain that a spectator will mention that this is only an optical illusion.

Now is the time to close your fist and re open it showing the three coins

Method :

You need a hidden coin in a classic palm position.( That is when a coin is squeezed between the fleshly base of your thumb and palm. )

This trick needs practise to get the rubbing of the coins to look that a third one is appearing, once you got it, it looks very effective .

source:bestmagictrick.info