You've the stack of coins of which 17 are turned heads up (the rest tails up). You don't know which coins are heads up (i.e., you don't know where in the stack the heads-up coins are located). Suppose that you're blindfolded and can't tell the heads/tails nature by feeling. Your job is to produce two stacks of coins with same number of heads up in each stack. You don't know how many coins there are in the original stack (but it's more than 17).