OK, now do this one:
Hey, hey, hey! What, am I sick or something? Well, yeah, I am. But, that's another story. Luckily, there is a much easier way to deal with this critter.
Let's look back at the ones we've already done:
See why it works? If you understand this thing, then you'll always know when you can use it -- and when you can't!
Here's the official rule: