God created the dog and said

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 twenty years.'

The dog said: 'That's a long time to be barking. How about only ten years and I'll give you back the other ten?'
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 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 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 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 again.

On the fourth day, God created humans and said:

'Eat, sleep, play, marry and enjoy your life. For this, I'll give you twenty years.'

But the human said: 'Only twenty years? Could you possibly give me my twenty, the forty the cow gave back, the ten the monkey gave back, and the ten the dog gave back; that makes eighty, okay?'

'Okay,' said God, 'You asked for it.'

So 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 do monkey tricks to entertain the grandchildren. And for the last ten years we sit on the front porch and bark at everyone.

Life has now been explained to you.