Spaceman lands on a strange
planet, and finds a beautiful alien (presumably female) stirring
a large pot. "What are you doing?" he asks.
"Oh! I'm making a baby!" she replies.
Spaceman says, "that's not the way we do it on Earth. Here,
let me show you."
The alien girl agrees, and they proceed to huff and puff away for
an hour. At the end of the hour, the girl gets up, dusts herself
off, and declares,
"That was very nice - but where's the baby?"
"Oh, that will take another nine months," the spaceman
replies.
"Huh?" the girls asks. "In that case, why did you
stop?"