http://www.georgehart.com/bagel/bagel.html

Mathematically Correct Breakfast

How to Slice a Bagel into Two Linked Halves

George W. Hart

[bagel0]

It is not hard to cut a bagel into two equal halves which are linked
like two links of a chain.


[bagel1]

To start, you must visualize four key points.  Center the bagel at
the origin, circling the Z axis. 
A is the highest point above the +X axis.  B is where the +Y axis
enters the bagel.
C is the lowest point below the -X axis.  D is where the -Y axis
exits the bagel.


[bagel2]

These sharpie markings on the bagel are just to help visualize the
geometry
and the points.  You don't need to actually write on the bagel to cut
it properly.


[bagel3]

The line ABCDA, which goes smoothly through all four key points, is
the cut line. 
As it goes 360 degrees around the Z axis, it also goes 360 degrees
around the bagel.


[bagel4]

The red line is like the black line but is rotated 180 degrees
(around Z or through the hole).
An ideal knife could enter on the black line and come out exactly
opposite, on the red line.
But in practice, it is easier to cut in halfway on both the black
line and the red line.
The cutting surface is a two-twist Mobius strip; it has two sides,
one for each half.


[bagel5]

After being cut, the two halves can be moved but are still linked
together, each passing through
the hole of the other.   (So when you buy your bagels, pick ones with
the biggest holes.)


[bagel7]

If you visualize the key points and a smooth curve connecting them,
you do
not need to draw on the bagel.  Here the two parts are pulled
slightly apart.


[bagel8]

If your cut is neat, the two halves are congruent. They are of the
same handedness.
(You can make both be the opposite handedness if you follow these
instructions in a mirror.)
You can toast them in a toaster oven while linked together, but move
them around every
minute or so, otherwise some parts will cook much more than others,
as shown in this half.


[bagel9]

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.

Topology problem: Modify the cut so the cutting surface is a
one-twist Mobius strip. 
(You can still get cream cheese into the cut, but it doesn't separate
into two parts.)

Calculus problem: What is the ratio of the surface area of this
linked cut
to the surface area of the usual planar bagel slice?

For future research: How to make Mobius lox...

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Note: I have had my students do this activity in my Computers and
Sculpture class.  It is very successful if the students work in
pairs, with two bagels per team.  For the first bagel, I have them
draw the indicated lines with a "sharpie".  Then they can do the
second bagel without the lines. (We omit the schmear of cream
cheese.) After doing this, one can better appreciate the stone
carving of Keizo Ushio, who makes analogous cuts in granite to
produce monumental sculpture.

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Addendum: I made a video showing how to do this.

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