When God created the dog, he said: "Sit all day by the door of your house and bark at anyone who comes in or walks past. For this, I will give you a lifespan of twenty years."
The dog replied: "That's a long time to be barking. How about only ten years and I'll give you back the other ten?"
And God saw that it was good.
When God created the monkey, he said: "Entertain people, do tricks, and make them laugh. For this, I'll give you a twenty-year life span."
The monkey said: "Monkey tricks for twenty years? That's a pretty long time to perform. How about I give you back ten like the dog did?"
And God, again, saw that it was good.
When God created the cow he said: "You must go into the field with the farmer all day long, work under the sun, have calves and give milk to support the farmer's family. For this, 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. How about twenty and I'll give back the other forty?"
And God agreed it was very good.
When God created man, he said: "Eat, sleep, play, marry and enjoy your life. For this, I'll give you twenty years."
But man said: "Only twenty years? Could you possibly give me my twenty plus the forty the cow gave back, the ten the monkey gave back, and the ten the dog gave back? That makes eighty years, okay?"
God agreed. And that is why for our 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 look like a wizened monkey and do monkey tricks to entertain the grandchildren. And for the last ten years, we sit on the front porch and bark at everyone.