It's decided to have a demonstration. The Martians go first. Four of them stand on top of each other, make whirring mechanical sounds, lights go off on their foreheads, smoke and bells, and...suddenly, a new Martian pops out.
"Fabulous, just fabulous, love the concept," say the earthlings. Then it's our turn. A suitable volunteer couple has been found, a bed cleared, and the couple goes at it while the Martians stand around taking souvenir photos. The pair finishes in a sweaty heap.
"Great, that was terrific, very novel," enthuse the Martians, "but one thing...er...where's the new earthling?"
"Oh, that," they are answered. "That happens nine months from now."
And the Martians ask, "So why were they in such a rush at the end?"