On the first day, God created the dog and 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 life span of 20 years."
The dog said: "That's a long time to be barking. How about only 10 years and I'll give you back the other 10?"
So God agreed.
On the second day, God created the monkey and said: "Entertain people, do tricks, and make them laugh. For this, I'll give you a 20-year life span"
The monkey said: "Monkey tricks for 20 years? That's a pretty long time to perform. How about I give you back 10 like the dog did?"
And God agreed.
On the third day, God created the cow and said: "You must go into the field with the farmer all day long and suffer under the sun, have calves and give milk to support the farmer's family. For this, I will give you a life span of 60 years."
The cow said: "That's kind of a tough life you want me to live for 60 years. How about 20 and I'll give back the other 40?"
And God agreed again.
On the fourth day, God created man and said: "Eat, sleep, play, marry and enjoy your life. For this, I'll give you 20 years."
But man said: "Only 20 years? Could you possibly give me my 20, the 40 the cow gave back, the 10 the monkey gave back, and the 10 the dog gave back; that makes 80, okay?"
"Okay," said God, "You asked for it."
So that is why the first 20 years we eat, sleep, play, and enjoy ourselves. For the next 40 years we slave in the sun to support our family. For the next 10 years we do monkey tricks to entertain the grandchildren. And for the last 10 years we sit on the front porch and bark at everyone.
Life has now been explained to you.