Let's say you have a standard bagel (one that is NOT pre-sliced). How can you cut this bagel into two interlocking rings? The rings must never be broken.
Ok Just to ensure that this question gets an answer I am posting this, and attributing it to community.
The basic concept is to consider the third dimension and 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.
Cut through the line ABCDA. Then turn the bagel over and make the same cut again. Here is a video showing how to cut the bagel.