My favourite motivation is to think of bank accounts. (A less monetary approach would be to think of a rising or falling temperature.) I think that anyone can predict the results of the following four operations:

* make a deposit in your bank account every day, and see your balance in a few days (positive times positive);

* make a deposit in your bank account every day, and see your balance a few days ago (positive times negative);

* make a withdrawal from your bank account every day, and see your balance in a few days (negative times positive);

* make a withdrawal from your bank account every day, and see your balance a few days ago (negative times negative).