Who tries to board a plane with a hand grenade — even a nonfuctioning grenade — in their carry-on luggage?

Jackass star Johnny Knoxville, that's who. Johnny, 38, was going through passenger screening at Los Angeles International Airport yesterday when the Transportation Security Administration officer who was inspecting Johnny's bag spotted the image of the grenade.

After the bomb squad arrived and determined that the grenade was harmless — it had no firing pin and no explosives — Johnny was allowed to board his flight to Miami.

And why was Jessica Simpson's ex carrying a grenade? It was a prop for a photo shoot, Johnny told officers, and he had just forgotten he had it.

Photo Credit: WireImage