How do slugs and snails eat?

Well, snails just kinda....eat. I am taking care of snails currently right now, and have fed them lettuce, and I have noticed that they kind of "bite" into the lettuce, as if they have teeth. You can hear them eating through the crunchy lettuce (crunch, crunch!). Once a snail bit me on the hand when I was letting it slither on. It doesn't hurt at all. I guess it was hungry at the time, but they don't bite me anymore.