But there is a tale that can help you to memorize it:
"Once upon a time… a negative (-) boy who couldn't decide (+ -) whether to go to a radical party. The boy was squared so he missed out (-) on 4 awesome chicks. The party was over at 2 a.m."
Try that with your students… and also have a look to this funny video:
What do you think of this lesson?
