"Grandad! Five take away two, take away one, take away one is one"
My Dad: 
DS is four and a half. I have had no end of ignorance from the man and if he could be arsed he would know that DS' "abilities" are not what they seem. He has heard it at school - probably once, and he has remembered.
Then again my enjoyment was short-lived because when he got over the shock he told me to limit DS' computer time as he is, "far too good for his age". He probably envisages my son bringing about a hacking-related nuclear holocaust 