A Mother, Father and Their Son Went to the Zoo

One day a mother, father and their son went to the zoo. Whilst walking along, they came to a pair of monkeys mating. The boy looks to his mum and says "Mum, what are they doing?" in which the mum replies, "They're making cakes dear".

They continue walking and they come to a pair of hippos mating. The boy, remembering what his mother said earlier, turned to his father and said "They're making cakes, aren't they dad?" and the dad replies with a little chuckle "Yes, son."

The next morning, the boy came downstairs and went into the kitchen to eat his cereal and he said to his mum and dad, "Mum, Dad, you were making cakes last night weren't you?". Uncertain of how he could possibly know, both parents asked how the boy knew.

The boy replied "Because I just licked the icing off the sofa"

EDIT:Changing to mother and father in opening sentence and other spelling mistakes.