Place three piles of matches (coins, or something small, that you can group together easily) on the table. Make one pile with 11 matches, the second one with 7 matches, and the third one with 6 matches.
You are to move matches so that each pile contains 8 matches.
You can add to any pile only as many matches as it already contains, and all the matches must come from one other pile.
For example: If a pile contains 3 matches, you can add 3 to it, no more no less.