What's the difference between theorectically and realistically

A young boy asks his dad: "What's the difference between theorectically and realistically?"

"Well son, go and ask your mother if she would sleep with the postman for $1million ..."

The little boy asks his mum and then goes back to his dad: "She said yes ..."

"Now go and ask your sister if she would sleep with the milkman for $2million ..."

The little boy asks his sister: "She said yes, but whats the difference between theoretically and realistically?"

"Well son, theorectically we are sitting on $3million ... realistically we are living with a couple of slags!"