You might remember from a math class ages ago that you can divide by zero. The problem is that a lot of students just hear "Zero... division... bad." It's perfectly okay to cut zero up into as many pieces as you like, but chopping a thing into zero pieces is problematic. This means another set of students will know that one way to have zero in a fraction is all right while the other way is bad, but they will forget which is which.
Christine Will, one of my fellow teachers at Laney, put this mnemonic up on the board during a talk and I think it's a good teaching tool.
