Bert sings about his favorite number.
I feel that Bert is often at his funniest when he gets all excited about one of his esoteric interests. This almost represents a role reversal, with Bert as the comic and Ernie as the straight man.
I love it when Bert lets his nerd flag fly. Let's be honest, even if we don't exactly share Bert's interests, all of us geeks and nerds can relate to being interested in something that "normal" people find boring. Bert is arguably the nerdiest character on Sesame Street, which makes him "one of us."