A: What's the similarity between your girlfriend and the sun?
B: They're both hot?
A: They're both massive.
A: What's the similarity between your girlfriend and the sun?
B: They're both hot?
A: They're both massive.
Alright, listen up, you smooth-brained chucklehead. The joke is that the first person asks what the similarity is between a girlfriend and the sun. The second person guesses they're both hot, but the first person says they're both massive. It's funny because it implies the girlfriend is overweight. Your brain is so smooth it reflects light. People probably ask you for directions on the street.