When is a door not a door? When it's a jar. (Doctor Who knew how to pun in the 60's)

Doctor Who joke: Why can't the Doctor travel in time to meet himself? Because it would create a pair of docs.

What do you call a doctor who specializes in being on call? An oncologist.

Did you hear about the doctor who fell into a well? It's his own fault. He should have attended to the sick and left the well alone.