Stop me if you've heard this one.
A college student needs money for books. His friend suggests donating sperm for a quick $50. He goes to the clinic, fills out the forms and takes a seat in the waiting room.
He sees an attractive young lady and sparks up a conversation. Turns out she's also in college and needs money for books. She's donating blood and expects to receive $10. She asks him what he's doing. He's a little embarrased but confesses that he's donating sperm. She asks how much they pay and he tells her $50. She's impressed that he gets so much for sperm.
The guy gets called to make his donation. When he returns to the waiting room, the pretty blonde is gone.
A couple weeks later, he finds himself needing money again and returns to the clinic. He sees the same pretty blonde in the waiting room and asks her "Are you here to donate blood again so soon?".
She just keeps her mouth closed, shakes her head and replies "mmm mmm".