This one's kind of gross.

Did you hear about the constipated mathematician?
He had to work it out with a piece of paper and a pencil.

And this is a supposedly true story from a doctor:
I was caring for a woman and asked, "So how's your breakfast this morning?" "It's very good, except for the Kentucky Jelly. I can't seem to get used to the taste," the patient replied. I then asked to see the jelly and the woman produced a foil packet labeled "KY Jelly."