It also doesn't mean that they can't understand your tone of voice or the expression on your face. When you're angry with them, they know. You can start to understand your pet just by being aound him and watching him over time. But you have to pay attention and think about what they're doing and what it might mean. It's really important to know a little about your pet before you pick him out or take him home.
Some pets love lots of attention and affection, getting petted all of the time and fussed over.
Some pets, like fish, shouldn't be touched at all, but they might like to have interesting plants or structures in their fishbowls or aquariums that they could swim around in. Or they might like the company of some other fish.