This problem has been adapted from one published in Scientific American. You can see the original puzzle and read more about this topic using the link at the bottom of this page.
Every year when I take out my Christmas lights, they are all tangled up, and it takes forever to sort them out. It always reminds me of an interesting area of maths called Knot Theory.
Of the four knots below, two can be untangled to make a simple loop. Which ones can be untangled?
