6*7 is one of the trouble makers that does need a little effort. I first of all give my kids a little "British Culture" lesson. If you say something is "at sixes and sevens" it means it's all messed up. And we all know the answer to life the universe and everything is 42. Well we do once we've had our British culture lesson. So if every thing is all messed up you put it right with the answer to everything. 6*7 = 42. You wouldn't think it would work if you have to give them the cultural reference first, but it does. And they enjoy yelling 42 at me when they see me in the hall