A prank built around a tiny piece of plastic
A hotel key card is a programmed credential: the lock reads the card and checks whether it grants access to that room. Without the cards, Rupert Grint and Daniel Radcliffe could not simply turn a handle and get back in; they had to wait in the hallway until the cards were returned or access was otherwise restored.
Why the joke worked
The prank’s punchline depended on a small inconvenience becoming instantly visible. The actors were left outside in their pajamas, with no elaborate setup required—just the key cards they needed to get back through the door. It was a fleeting backstage-style joke, but one with a clear practical consequence: for a hotel guest, a key card is the difference between being dressed for bed and being stuck outside the room.
