God created mule and told him, "You will work constantly, living for 40 years." The mule wanted only 20 years. And it was so. Then God created dog and told him, "You will guard the dwellings of man for 25 years." The dog wanted no more than 10 years. And it was so. Then creating the monkey God told him, "You shall act like an idiot for 20 years." The monkey wanted 10 years. And it was so.

Finally, he created man and told him, "You will dominate the earth for 20 years." the man responded, "20 years is too little." Give me the 20 years the mule refused, the 15 years the dog refused, and the 10 years the monkey rejected."

So, God made man to live 20 years as a man, then marry and live 15 years as a dog guarding the house, then in his old age to live 10 years as a money, amusing his grandchildren.