I once got laughs from friends in a maths class (mumblemumble years ago), by describing someone as "So far up his own arse, he's a Klein bottle"*

W


* A Klein bottle is a vessel where the inside becomes the outside.

"A young matematician called Klein
Thought the Moebeus strip was devine
He said "If you glue
"The edges of two
"You end up with a bottle like mine".