In an illustrated grammar for children I saw a picture of a cat sleeping on a ball of string (wool?) and one of the characters says: 'Look, Kitty is sleeping on the ball.'
Could they use only 'ball' for 'ball of string/wool'? Perhaps they used only 'ball' because the illustration made it clear what kind of ball it was?
Secondly, I read 'It could be a lesson in patience.'
Shouldn't it be 'a lesson ON patience'?