  The classical trefoil is famous for having a three-colouring which
distinguishes it from the unknot. The three-colouring is also notorious for not
distinguishing the right handed from the left handed trefoil. However with a
bit of tweaking the three colours can also be used for this task. What lies
behind the method is a new operation on biracks called doubling which converts
the 3-colour quandle into a biquandle. Colouring with this biquandle
distinguishes the right handed from the left handed trefoil. Equivalently it
defines an element of the homology of the quandle or biquandle classifying
space.
