[She pauses a brief moment, considering how to best answer this, before deciding he'd probably work best with just a straightforward, direct explanation.]

Unicorns, yes. Well, Alicorns too, but there are only three of those in existence. It's the horn, you see. Since it's directly connected to the brain, it serves as a superb magical catalyst, allowing us to focus our own energy with that of the outside world and cause magical phenomena with comparative ease. The feeling of a familiar spell is very similar to giving orders to one's limbs, which is why telekinesis is the one spell that pretty much all of us know.