You are standing in front of a table, where there are 100 coins. Out of the 100, you know that 90 are placed where heads are up, and 10 are with tails up.
You are blindfolded, and you are requested to separate these into two groups, in which there will be the same amount of coins with tails up. You obviously cannot "feel" the coins, however you can flip them (without knowing whether they were heads or tails prior).
How would you do that?