On the first day God created the 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."
The dog said, "That's too long to be barking. Give me ten years and I'll give you back the other ten." So God agreed.
On the second day God created the monkey. God said, "Entertain people, do monkey tricks, and make them laugh. I'll give you a twenty-year life span."
The monkey said, "How boring, monkey tricks for twenty years? I don't think so. Dog gave you back ten, so that's what I'll do too, okay?" And God agreed.
On the third day God created the cow. 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."
The cow said, "That's kind of a tough life you want me to live for sixty years. Let me have twenty and I'll give back the other forty." And God agreed again.
On the fourth day God created man. God said, "Eat, sleep, play, marry and enjoy your life. I'll give you twenty years."
Man said, "What? Only twenty years! Tell you what, I'll take my twenty, and the forty the cow gave back and the ten the monkey gave back and the ten the dog gave back, that makes eighty, okay?"
"Okay," said God, "You've got a deal."
So that is why the first twenty years we eat, sleep, play, and enjoy ourselves; for the next forty years we slave in the sun to support our family; for the next ten years we do monkey tricks to entertain the grandchildren; and for the last ten years we sit on the front porch and bark at everyone.