There was a middle-aged woman next to me in the Starbucks line (I literally went out of my dorm in College, went to Starbucks, just to perform this act of kindness). I asked her name and then offered to pay for her coffee. She smiled and said, "thank you, it's very nice of yours!"
Later, she greeted me goodbye with, "Thank you so much for the coffee! I'm gonna pay it forward." I didn't even have to tell her. She said it herself! :D