I was out with my wife a few years ago and we were in a Marshall's.
As we were waiting in the checkout line a lady was behind us with her son in the seat of the shopping cart. The boy had a ball and tossed it out of the cart towards me. I picked it up and returned it to him and got back in line. He tossed it again. Again I returned it. We did this about 5 times, all the time smiling at each other. Finally the mother said to him, "that's enough, the nice lady doesn't want to play anymore". We smiled at each other and off I went to pay for my items.
It was very heartening to interact with them like that and kept me smiling all day long.