On the first day God created cow. And God said, "You must go to the field with the farmer all day long and suffer under the sun, have calves and give milk to support the farmer. I will give you a life span of sixty years."
Cow said, "That's kind of a tough life you want me to live for sixty years. Let me have twenty years and I'll give back the other forty." And God agreed.
On the second day God created dog. And to dog, God said, "Sit all day by the door of your house and bark at anyone who comes in or walks past. I will give you a life span of twenty years."