Penny locking a door, otherwise known as pennying up a door, can be an amusing practical joke to play on a friend or family member. To penny lock a door you simply jam pennies between the door and the hinge from the outside, which prevents the door from opening from the inside. If done properly, the only solution is to remove the door from the hinge. While penny locking a door can work, it will only work with certain door jams.
Examine the door you want to lock. If the door frame has a lip that extends over the door, as many office building doors have, it will be difficult to jam the pennies between the door and the jam. If the frame has no lip, get your pennies ready.
Select the appropriate number of pennies for the space between the door and frame. Most doors will need four or five. Use the maximum number of pennies you can fit in there to ensure that the door will stick.
Wrap your chosen stack of pennies in tape. By wrapping the pennies, you ensure they will not separate when the door is open, thus ruining the joke. Also, wrapping the pennies makes them easier to jam in between the door and frame, and ensures no permanent scuff marks will be left on the door frame.
Hammer the pennies between the door and the frame. There is disagreement as to whether it is better to hammer the pennies on the hinge side or the doorknob side. To ensure the door is jammed, go ahead and hammer in a stack on both sides. The important thing is to create enough friction between the door and pennies that the door can't be opened.
Knock on the unsuspecting person's door and laugh heartily as they try to open it. To be nice, help them put the door back up after they have taken it off of the hinges, which is what they will have to do to get out of the room.