3.14 Cakes

This is an anti-joke. The setup leads you to expect a punchline involving the mathematical constant pi (π). Instead, the answer is simply 'Fat,' which is a literal and non-humorous response to eating a lot of cake. The humor comes from the unexpected lack of a clever joke and the fact that it's just a simple and obvious statement. This subverts expectations since you would anticipate a play on words with 3.14 and pi. The last sentence highlights the anti-joke nature.