Q: Did you hear about that mathematician with constipation? A: He worked it out with a pencil.
A doctor walks into a room full of patients at a mental institution, takes out a pen, and draws a door on the wall. He then tells all the patients that whoever wants to escape, should use that door. Immediately they all rush towards it, but of course cannot go through. However, one patient sits still in the back with a smile on his face. He has not moved at all. The doctor thinks he must be cured. He then asks the patient why he did not rush to the door, and the patient whispers, "They don't know that I'm the one who has the key."
It's hard to explain puns to kleptomaniacs because they always take things literally.
Q: Did you hear about the flasher who was thinking of retiring?
A: He decided to stick it out for one more year!
Q: What's the difference between a lawyer and a leech?
A: After you die, a leech stops sucking your blood.