A Monkey Mistake - Is the Slope Negative or Positive?

One problem I find that students have with interpreting linear graphs is that they often mistake a negative slope for a positive slope and vice versa. Knowing this AND knowing that if I can make them laugh, I can make them learn (which is the philosophy behind this blog), I have used this video for this purpose.

Now if they confuse a negative and a positive slope, I can tell them they made a "monkey" mistake!