Slap Fu

I asked the boy how his day at school was.
' Me and bob had a dead arm fight and then me and jack arm wrestled.'
'No I mean, how was the class bit- where you learnt stuff?'
'OK.'
'What did you do?'
'Usual.'
'Like what?'
'Can't remember.'
'So you didn't learn anything?'
'Yes we did.'
'If you can't remember, then you didn't learn it.'
'Oh, we had IT.'
'That's good, what did you do?'
'Me and Dick are way ahead and the supply teacher let us play games.'
'Did you have a GOOD day?'
'Yeah it was great. I invented a new martial art. It's called Slap Fu. You put one hand behind your back and wave your arms about and score points if you manage to slap someone in the face.'
'Oh.'