You know show a pile of keys and explain that only one of these keys will open the lock. You further explain that everyone gets a free choice of any key and if they open the lock - they win the money! The keys are now mixed up and each spectaator chooses one key...leaving only one behind for the you.

Each person attempts to open the lock and one by one each spectator fails. You take the key left behind and open the lock!

A great routine with lots of presentational possibilities.