Analogies, yeah!

So many new players/tanks seem to have trouble understanding the tooltip for Taunt, that it's not effective if the mob is already attacking you.

Then I thought of it like healing: taunting when you already have aggro is like healing a person who's already at full health. They can't get any healthier!

You could even say it's like attacking a target that's already dead - it's not going to get any more dead!