You display a small bag that has about 15 coins inside it. The spectator dips their hand inside the bag, and removes any coin (let us assume it is an English 10p). The spectator remembers the coin and replaces it in the bag. The spectator takes the bag and shakes it to mix the coins. You reach into the bag and remove a coin, and it is the same coin as the one the spectator selected. You offer to explain how you did it. The bag is emptied onto the table and every coin is the same, they are all 10p coins...... Easy!