"You can describe the configuration of a unicycle on a sidewalk using three coordinates (x,y,t) where t is the angle that the direction the wheel makes with the x axis. The motions of the unicycle are constrained to moving in the direction the wheel is pointing. This is an example of a 'contact structure,' and a path that obeys the constraints is a 'Legendrian curve.' If the curve returns to its starting spot it is a 'Legendrian Knot.'" - Josh Sabloff
Credit to nwbx for the unicycle model