Penny Hofstadter: I had to stop at Sheldon’s and help him solve string theory.

Amy Farrah Fowler: What?

Penny Hofstadter: Yeah, turns out the answer’s knots.

Leonard Hofstadter: That’s cute, but you can’t have knots in more than four dimensions.

Penny Hofstadter: Mm, you can if you consider them sheets.

From The Big Bang Theory – Season 11 Episode 13: ‘The Solo Oscillation’ (11×13)