Sex!

The first rocket ship arrived on Mars, and the astronauts were welcomed with great Martian joy and curiosity. In the course of the interviews that followed over the next several days, the interrogators came to the topic of sex.

The commander of the rocket began an explanation, but the Martians couldn't believe what he was attempting to explain, so he asked Major Julie, the medical officer, to help him with an actual demonstration. The Martians watched the demonstration with fascination, and when the commander and the medical officer had finished, they asked, "And then what?"

"Well, then, we wait 9 months and see what happens."

"NINE MONTHS? Then why were you in such a rush to get it over with here at the beginning?"