Use the mouse to move the black box. When the black box is over the red one, tap space. Keep doing this until you've cracked the safe. But, be careful when you tap space, because if the black box is not over the red one, you add 1 mistake. When your mistakes are out, you lose. Also, if you do not open the safe in 30 seconds, you lose.
For every safe you crack you regain 3 chances.