George Hart shows us how to cut a bagel such that it ends up as two linked bagel-rings. His knife follows the surface of a two-twist Möbius strip, making use of the initial bagel's ring shape to tease the second ring out of it. Hart then proceeds to show us how to toast it, and asks the tantalizing question: how much more cream cheese can you get on this cut bagel, versus a traditional cut? Not being a mathemagician, I'll let y'all talk that one through in the comments.
My favorite YouTube comment: "Gödel, Escher,? Bagel." (Ahem.)
See written instructions: Hart's Mathematically Correct Breakfast and Mathematically Correct Breakfast II, the latter featuring two ways to make your bagel into a trefoil knot (not a video, but including beautifully bagelicious photography).
(Via It's Okay To Be Smart.)