There is a story about a guy who makes a point to be kind to his taxi driver, giving him a sincere thanks and a generous tip. The guy reasons that, if the taxi driver is then inspired to be kind to each of his passengers, which can be as many as 100 per day, and every passenger goes on to be nice to the next person they meet, who is then kind to another person, and so on, then the guy’s initial kindness could theoretically brighten the day of thousands of people in a big city.