[ska]

Q: How can you tell there is an topologist in the cafeteria?

A: Because they can't tell their coffee cup and donut apart.

[zacinbusiness]

I up voted this because it made me laugh, even though I don't understand.

[ska]

Fundamentally they both have only one hole through them, so under the right sort of deformations you can turn one shape into the other: They have the same topology.