A joke from last night

I heard this one from a fellow party goer last night: A father is taking his young son for a walk through the park. The son sees a couple of dogs mating. The son asks his dad what they are doing. The dad replies, "They're making puppies." That evening the son catches his parents together in their bedroom doing what parents sometimes do. The son asks his dad what he is doing. The dad says "We're making you a brother." The son cries out, "I don't want a brother. Turn mom over, I want puppies instead!"