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 - "Hmmm, 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."