Three guys stumbled out of a pub. The first guy, Bob, saw something flying through the air. He was mesmerised by this flying object, but he couldn’t tell what it was.
So he simply shouted, “Look, a UFO,” as he stumbled after it.

Bill and Brad followed, and the three comrades watched as the Unidentified Flying Object started to spiral out of control.
“Oh no! The UFO’s falling,” shouted Bob.
Bill hearing his exclamation was quick to correct him, “Don’t you mean U-F-O?”
A confused Brad turned to Bill as Bob kept chasing the UFO.
“What?”
“Unidentified Falling Object,” Bill replied as he passed out laughing.

Brad slowly caught up with Bob and found him standing over the grounded UFO.
“The UFO,” lamented Bob.
“It’s not a UFO anymore,” replied Brad.
“Yes it is. An Unidentified Flat Object,” said Bob as he passed out laughing.

Brad, the dumbest of the lot, continued aimlessly, but soon passed out laughing his head off as he finally got the joke.