One morning in December of 2012, at the Canadian drive-through window of a Tim Hortons coffee shop in Winnipeg, a customer paid for her order and then picked up the tab for the stranger in the car behind her in line. Then that customer paid the bill for the following customer in line - and so on, for the next 226 customers, in a three-hour sequence of spontaneous generosity.

News outlets have reported the emergence of many such chains in a variety of restaurant drive-throughs, like this donut shop, and others throughout North America.

Social scientists have conducted experiments demonstrating that the effect of a single act of kindness can in fact ripple through a social network, setting off chains of generosity

(READ the story from the New York Times)