Person 1: What's the integral of 1/cabin?
Person 2: A natural log cabin.
Person 1: No, a houseboat – you forgot to add the c!

Explanation follows in case you haven't gotten the wonderful joke at first try:

The first part of this joke relies on the fact that the primitive (formed when finding the antiderivative) of the function 1/x is ln(x). The second part is then based on the fact that the antiderivative is actually a class of functions, requiring the inclusion of a constant of integration, usually denoted as C — something which many calculus students forget. Thus, the indefinite integral of 1/cabin is "ln(cabin) + c", or "A natural log cabin plus the sea", ie. "A houseboat".

You can't stop laughing right? I know it's tough...