Stingler

The Stingler is an item used in Mario & Luigi: Bowser's Inside Story and its remake. It appears in Pump Works. While Mario and Luigi are searching for Princess Peach, they discover a DNA-like door with an odd key slot. Later, they find the Stingler, and it turns out that they need it to enter through the door. After being inserted into the door, more sections of Bowser's body become accessible, including the Flame Pipe where it is believed Peach may be located.

Trivia

 * Before the Stingler is used, the Emoglobin refers to it as the "bzzzt".