An alien and a man were sitting next to each other in a bar. The alien was constantly poking the guy's cheek and saying, "zzzt!" Finally, the guy got so mad at the alien that he said, "If you do that one more time, I'll chop your penis off!''
Again, the alien poked his cheek and said, "zzzt!"
The guy said, "Okay, that's it!" He got up, grabbed the sharpest knife he could find, and pulled down the alien's pants. But he was astounded to see nothing there.
He then said, "Well, if you don't have a penis, how do you have sex?"
The alien just smiled, poked the guy's cheek, and said "zzzt!"