What's the nicest thing a complete stranger has ever done for you?

This summer I moved to NYC for a few months and anyone who’s ever been there knows how intimidating the place can be to a newcomer, especially considering that New Yorkers have a bad rep for being rude and self-absorbed. So it came as a total surprise when I was heaving my very large and heavy suitcase up a huge set of steps in a Times Square subway station, trying to figure out which train to take, when suddenly a complete stranger going in a totally different direction from mine saw me, stopped in his tracks, went out of his way, and helped me carry it the rest of the way up. For a moment I thought he might be one of those guys who go around giving directions and asking for “donations” but he didn’t even give me a chance to thank him properly because he was on his way again as soon as I made it to the top of the stairs. Ever since that day, whenever I see someone struggling with something heavy, I go and help them.

Has a stranger ever done something nice for you, something that left an impression? And how do you pay it forward?