There was a family: mum, dad, brother, and two twin sisters the brother being the eldest child.
One day the mum decided to take the two twin daughters out to the zoo. The girls saw two monkeys having sex and asked: "Mum what are those monkeys doing?", the mother replied: "Oh, they're just making cakes."
The next day they went to the park and saw two dogs and asked: "Mum, what are those dogs doing?", the mother replied: "Oh, they're just making cakes."
The following day the family is at home and the two daughters go upstairs and run down to tell their brother "Mum and dad have been making cakes", he replies "How do you know?" they answer "because we just licked the icing of the bed."