UPDATE: The Mobius Doughnut!
A bagel is a geometric marvel all on its own—leave it to an MIT Ph.D. and mathematically minded sculptor to up the ante.
Professor and artist George Hart figures out how to cut a bagel into two congruent halves—they "pass through each other's holes, like two links of a chain." One bagel. Two halves. Unbroken. Interlocking.
Impossible, you say? Not when "the motion of the knife follows the surface of a two-twist Möbius strip." Yes, in a single long knife cut, you can transform an everyday bagel into two linked halves. And the best part?
It is much more fun to put cream cheese on these bagels than on an ordinary bagel. In additional to the intellectual stimulation, you get more cream cheese, because there is slightly more surface area.
Update: This works! This really, really works. If you've never cut a bagel into two interlocking halves (that is, all of you), grab a knife and try it. You'll never look at a bagel the same way again. (Thanks, @harveyj!)
Update #2: Check out the Mobius Doughnut!