Baking a cake.

One night little johnny woke up and went to get a glass of water.  As he walked by his parents room he heard them having sex.  The next morning he asked his mom what they were doing and she replied "we were baking a cake honey."  A few weeks later Johnny's older brother snuck his girlfriend over to the house.  The next day Johnny said to his mom "Brother and his girlfriend were making a cake last night." she replied "How do you know that?" Johnny answered "Well when they were done I went into his room and they had spilled frosting on his bed so I licked it up."