All lions are animals. Therefore, all lions are cats. For an argument to be valid, it must be the case that IF the premises are true, the conclusion must also be true. But just because all cats are animals and all lions are animals doesn't mean that it's got to be true that all lions are cats. In some other possible world, lions could actually be members of the dog family.
All cats are animals.
All frogs are animals. Therefore, all frogs are cats. Therefore, it is invalid. Unsound, because it's invalid. No cats like seafood.