Key
From Twinsuniverse
Keys are used for opening doors and gates. Twinsen usually obtains them by either searching in plants or garbage bins, or by killing a guard keeping the key. He can take a key by either walking over it, or by throwing his magic ball at it, as it is attracted to keys.
If Twinsen goes out of a room, he loses all the keys in his possesion, regardless of whether the door he passes requires a key or not. The number of keys Twinsen has is indicated in the control panel.