Digital story joke

An old man has 17 donkeys and three sons. After his death, he will give his 17 donkey to one half of his eldest son, one third of his second son and one ninth of his third son. How come?

Add a head first, which is divided into 18 heads. Half of 18 is 9, one third is 6 and one ninth is 2,9+6+2 =17. Just finished.