Can teach modals 'might', 'could' or 'may' for unknown possibility.  'Probably' for a more likely possibility.

Motives, for me, immediately sparks the debate of whether there is such thing as 'altruism'.  Even weak pupils can bang this one out.

I would start the debate (since I work in a religious Jewish school) asking why we do mitzvot (deeds prescribed by the Torah) (you can talk about 'good deeds').  Is it to get 'spiritual points', is it habit, is it because we like feeling connected to traditions/community/family, is it because being nice gives up satisfaction, is it social pressure, is it 'to be seen as religious' by others or is it because it is God's command.  Finally, is an altruistic reason better than doing something for benefit. If the pupils cannot visualise this, I'd use the example of giving to charity.  (I'd like to hear your comments too). 

Then, I would present real examples of acts of kindness and ask the pupils if there are any other possible motives they can think of.

For example, walking an old lady across the road, giving to charity, respecting parents (and teachers - hammer it home) and being someone's friend.