Fake door

A Trapdoor (also known as a Fake Door) is a special trap that resembles a real door. Trapdoors are first encountered during the events of Luigi's Mansion, and later appear in Luigi's Mansion: Dark Moon. They're designed to confuse and injure Luigi as he progresses through both games.

Luigi's Mansion
In Luigi's Mansion, Trapdoors are found in various locations on different floors of the mansion. If Luigi happens to turn the knob of a Trapdoor, the door springs open and flattens him against the wall, causing him to lose ten HP and a few Coins. A ghost can be heard laughing after Luigi is smashed. Sometimes, a trail of conveniently placed Coins leads straight to a Trapdoor to catch Luigi off guard.

The only way to defeat a Trapdoor is to set it on fire; while real doors don't catch on fire, Trapdoors go up in flames and disappear. However, if Luigi enters two different rooms and returns to where the Trapdoor was, it will have reappeared. During the blackout in the game, the ghosts increase the amount of Trapdoors in the mansion and replace certain real doors with Trapdoors, including those leading to the Storage Room, the Projection Room, the Kitchen, the Parlor, and the Anteroom.

There are several ways to tell a Trapdoor apart from a real door. For instance, Luigi glances at Trapdoors as he walks by them, but doesn't look at real doors. Additionally, Luigi can check his Game Boy Horror; real doors appear on the map, while Trapdoors don't. Luigi can also vacuum a door with the Poltergust 3000; if it rattles, it's real, if it doesn't, it's a Trapdoor. Another way to differentiate between the two involves looking for doormats; real doors have doormats under them while Trapdoors do not (though in some cases, a real door, like the one leading to the Fortune-Teller's Room, won't have a doormat either).

Luigi's Mansion: Dark Moon
In Luigi's Mansion: Dark Moon, Trapdoors are found within the five mansions in Evershade Valley: Gloomy Manor, Haunted Towers, Old Clockworks, Secret Mine, and Treacherous Mansion. They serve the same function as they did in the first game, though Luigi now loses only five HP when hit by one, and doesn't lose any Coins. Additionally, ghosts are no longer heard laughing after Luigi gets flattened by a Trapdoor. Just like in the first game, Luigi can use his map &mdash; this time on the Dual Scream &mdash; to determine whether a door is real or not, as Trapdoors will not show up.