All whales are mammals. Some mammals live on land. Therefore, whales live on land. It is unsound, because it is invalid. All whales are mammals.
All mammals nourish their young with milk.
Therefore, whales nourish their young with milk. If the premises are true, then the conclusion is guaranteed to be true, too. Sound, because it is valid AND the premises are true. If a student passes Philosophy 101, he or she will receive 3 credit-hours towards the Humanities requirement. If a student enrolled in Philosophy 101 works hard, he or she will pass the course.