Okay, its pretty simple, but just in case anybody needs to know how it works, you just put a row of signs attached to each other behind a block that's on top of a chest. Now if this is on an adventure map, and the player sees the sign FOOD written across the chest, they will open it for sure. This breaks the signs all the way down the row, dropping them on wooden pressure plates, that then power the TNT. It does not NEED to blow up, it could for example trigger a dispenser to shoot a whole bunch of creepers out or something, but that's up to you. NOTE This must be backed up to a wall as shown, or the player will easily disarm it.