An endless source of amusement for programmers is the observation that the directions on shampoo, "Lather, rinse, repeat," are an infinite loop because there is no iteration variable telling you how many times to execute the loop.
In the case of
countdown, we can prove that the loop terminates because we know that the value of
n is finite, and we can see that the value of
n gets smaller each time through the loop, so eventually we have to get to 0. Other times a loop is obviously infinite because it has no iteration variable at all.