What is a nice interaction you’ve had with strangers?

Have just perused a thread on this forum about situations where people have been staggeringly rude so wondered if anyone had experienced the opposite? I’m thinking of one time when I was in our local supermarket (Booths). Just having the usual conveyor belt conversation (‘that cake looks nice etc) with the guy who was next in line for the till. When he was putting his items through he noticed that the woman who had been ahead of him had left her card in the reader. So he plucked it out, gave me a £20 note and asked if I would pay for his groceries while he hared after her. I packed and paid for his items. He came back having reunited the card with its owner. I handed him his change and we all parted full of bonhomie!