A figure eight knot
The figure eight knot (also called the savoia knot) is the simplest knot after the trefoil. You can compare the image with this other, that represents another diagram of the same knot. Viceversa, diagrams that differ from this one for a switch of the crossings (up/down), can produce a trivial knot or the trefoil knot.
The image has been realized with the program KnotPlot.