[...] This animation plots sin, cos, and tan.  Tan is cool because it has asymptotes (loosely: straight lines that the equation approaches but never reaches – kind of like the Cooper situation).  This is because for angle a, tan a = opposite/adjacent = sin a / cos a … but WAIT!  cos a sometimes comes out to 0!  We see that in the graph whenever the cosine line touches the x-axis.  Well, as you probably recall from algebra, you can’t divide by 0.  The answer is just undefined.  So when cos a hits 0, you’ll see the tan line jump to a remote y coordinate – in other words, it appears again at the edge. [...]