A man and a woman were waiting at the hospital donation center.
Man: "What are you doing here today?"
Woman: "Oh, I’m here to donate some blood. They’re going to give me $5 for it."
Man: "Hmm, that’s interesting. I’m here to donate sperm, myself. But they pay me $25."

The woman looked thoughtful for a moment and they chatted some more before going their separate ways.
Several months later, the same man and woman meet again in the donation center.
Man: "Oh, hi there! Here to donate blood again?"
Woman: [shaking her head with mouth closed] "Unh unh."