Minecraft wiki entry for Creating_a_challenge_map, associated text: Put attractive blocks such as diamond/gold on a pedestal. 13 blocks below, place a piston that fires every so often (1 second is good.) Because pistons can push a maximum of 12 blocks, nothing will happen until the player takes the block, because the piston will extend if they don't put another block (like sand) in its place. Connect the piston to a BUD switch to activate some traps and hope the players remember to grab their hat!