A gentleman is undergoing a vasectomy.  During the delicate operation, one of his testicles falls onto the floor and before the nurse can pick it up, the doctor steps on it.

The doctor tells the nurse,  "Don't worry, we can replace it.
Get me a very small onion."  She does and the doctor replaces the missing ball with the onion.

A few weeks later, the patient stops by to see the doctor, who asks him what seems to be the problem.

"Well, it's like this," the patient replies.  "Every time I take a
piss, my eyes water.  Every time I come, I get heartburn,
and every time I pass a Burger King, I get a hard-on!"