If dogs and cats had thumbs, it would change what it's like to have them as pets. They would be able to do more things with their hands, like open doors or turn doorknobs. Owners might find it harder to keep pets out of places they aren't supposed to go. Pets with thumbs could also learn to do more tasks around the house that usually require hands. Overall, pets with opposable thumbs would be more independent but possibly more mischievous.