A father is jewelry maker and he has 10 sons. He gives each son 20g of gold and tells them to make 10 gold rings each. One son cheats on his rings; he uses only 10g of gold to make 10 rings and pockets the the other 10g. Now the father has collected all 10 rings from each son and wants to see if the one son cheated. He can only use and read the scale ONCE. How does the father figure out who cheated?