A trefoil knot is the simplest type of non-trivial topological knot. See the Wikipedia page for Trefoil Knot.
The most interesting thing about this shape is that it has only one face and only one edge! Yet it still has volume!
For STEP, IPT and SAT downloads, see Trefoil Knot (Square) on GrabCAD.
See on with a circular circular cross-section here.
See the images for my own makes.