Nobody is quite sure when this drive-thru ‘pay it forward’ trend has become popular, but by now, we are no strangers to it. If you are, well, here is a quick explanation. It all starts with one person who wants to do something small to make a stranger happy. They decide to do that by going to a drive-thru and asking the barista to charge them for the order of the car behind them, so they get their order for free. A small gesture, that usually doesn't cost that much, to make someone else a littl…