Mathematically Challenging Bagels
Robert Krulwich
Thursday, November 8, 2012 at 11:06 AM
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# A guide to making a Mobius bagel. Cut along the black line.

## George W. Hart

All you need is a bagel, a knife and a high score on your math SAT, and you can do this (unless you're me): You can transform a single bagel into two intertwining, connected parts, one twisted through the other. In other words, a Mobius bagel. Watch and learn.

# Mathematically Challenging Bagels

Surgically, this will be complicated. Mathematically, it will be elegant. What we are going to do is take an ordinary bagel, and rather than cut it in half, we are going to turn it, delicately, into two intertwining, interlocked bagel parts, connected, unbroken, one twisting through the other. In other words, a Mobius bagel.

This is what mathematicians do on lazy afternoons. It's also a way to have more bagel surface to slap cream cheese on, says math teacher and sculptor George Hart (who's so skinny, he couldn't do this often.)

Here's how it's done. If you had the hands of a surgeon and the brain of Pythagoras, you would take a knife and carve a gentle, 360-degree slice that dips down and comes back up in a perfect, interior swirl. This video demonstrates the ideal cut, but remember it's slicing an ideal bagel, with no crumbs, no imperfections, so this will never happen in real life.

But many in real life have tried. Since George Hart published his cutting scheme a few years ago, high schools now regularly ask students to make Mobius bagels (even if the term "Mobius" isn't quite right, because a true Mobius twists; these break).

Believe me, it isn't easy. I just tried, and the bagel fell apart because I couldn't get the knife to make the final near-to-the-surface pass without screwing up. But that's me. Some people have a gift.

Take "Kirill," a college freshman who chose what appears to be a raisin bagel (which is crazy — adding random, lumpy obstacles is like throwing rocks onto a skating rink), and for his utensil, I think he used a cheap, plastic cafeteria knife, and yet, watch what he does.

One day, Kirill is going to be a brain surgeon.

Me? I'm in radio. My bagel's in tatters. I'm covered in crumbs. But I've got cream cheese, so I'll be fine. A little ashamed, a little chubbier, but fine.

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