Muggles' Guide to Harry Potter/Magic/Decoy Detonator

Overview
Decoy Detonators are products of Weasleys' Wizard Wheezes. They are small devices which scurry off into shadows and then make a noise, thus decoying people away.

Extended Description
We first see Decoy Detonators when Harry, Ron, Ginny, and Hermione, along with Mr. and Mrs. Weasley, visit the Twins' new joke shop in Diagon Alley. Fred, giving Harry the tour of the shop, takes him into the back room where the more serious magical items are kept, and there shows him the Decoy Detonators, saying they can't keep them on the shelves.

When the Trio are planning to break into the Ministry in Harry Potter and the Deathly Hallows, Decoy Detonators are one of the things on Hermione's list. Harry then uses one of them to distract a room full of witches and wizards so that he can enter Umbridge's office undetected. The Detonator scuttles silently to the far side of the room, then blows itself up, making a sharp noise and a cloud of smoke. The witches and wizards examine it for a long time, finally deciding that it is an escapee from the department of Experimental Charms.