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?"