Mongoose

The drunk was floundering down the alley carrying a box with holes on the side. He bumped into a friend who asked, "What do you have in there, pal?"
"A mongoose."

"What for?"

"Well, you know how drunk I can get. When I get drunk I see snakes, and I'm scared to death of snakes. That's why I got this mongoose, for protection."

"But," the friend said, "you idiot! Those are imaginary snakes."

"That's okay," said the drunk, showing his friend the interior of the empty box, "So is the mongoose."