Knowledge is Power
In an effort to show us that making a lot of noise is not always a good thing in a dungeon, we are almost caught by surprise by a small band of wandering trolls when some fool is trying to chisel a stone door off its hinges. Even though the Paladin couldn't hear his own thought processes we are not totally surprised and leap in to battle.
Not having fought trolls before we are unsure of what abilities and features these monsters have, so we ask our Mage, the fount of knowledge that he is, what he knows about them as we try to beat them down. The Mage thinks for a bit, rubbing his chin in a splendidly scholarly pose, before telling us that if the trolls strike and hit with both claws in a single attack then they can rend and do even more damage.
That is certainly good to know, but I had found it out myself, mere seconds before the Mage enlightened us, the painful way.
Next week, I inform the party that there may be a trap, after picking up a stray gold coin and having water start to flood the room. I can make Knowledge checks too!