Trefoil disguises: http://shpws.me/Tk5w Unknot disguises: http://shpws.me/Tk5e The idea to make optical illusions with knots came from a project one of my students, Austin Elliott, did for the "3D printing and math" class I teach at Oklahoma State. In Austin's design the knot could cast shadows in two directions, one of which looked like the shadow of a trefoil knot, and the other looked like the shadow of a figure eight knot. The shadow of a knot loses the crossing information - in my versions you can see the crossing information (or at least, the crossing information of the disguise), but it seems harder to make the knot look nice and smooth from a side view as well as the "disguise" view. Secret evil extra puzzle: Starting at 1:36, why is there a faint extra image of the knot in the mirror?