I've been knotty

In the mathematical field of topology a knot is a closed curve in space, like a tangled loop of string. The simplest knot is the trivial “unknot” which is just a loop. The simplest nontrivial knot is the trefoil knot, like the one in the upper left of the photo.

Topological knots are considered “topologically equivalent” if you can transform one into the other by pushing and pulling on the string without having to cut it.

Imagine that you take the string and lay it flat on a table; unless it's the trivial “unknot” it will have to cross itself some number of times. For example, the trefoil knot crosses itself 3 times. Knots can be categorized by this number of crossings. It turns out that there's only one knot that has 3 crossings and one knot that has 4 crossings, while there are two knots that have 5 crossings. The models here are instances of those four knots.

As you can see from the Rolfsen table of knots there are lots more. Maybe I'll make some more models if I feel knotty again.