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)