How can this be?(智力题)

问题描述:

How can this be?(智力题)
Two fathers and two sons are walking in a street.They buy four cakes.Each of them eats a cake and yet there is a cake left.How can this be?

A grandpa,a father and a son
爷爷爸爸儿子
也是两个爸爸两个儿子