I think I've told this story here before but it's perfect for this thread.
At my first "real" job I was working the cash at a fast food restaurant. A man came to my line and handed me a written list - this wasn't unusual so I didn't think anything of it, but we were out of one of the items, so I told him so. When I looked up for his response, he looked miserable, then pointed at his ears, shaking his head, indicating that he couldn't hear.
So I signed, "Do you know sign language?" I tell you, the light that came across his face! He nodded, I flung the list over my shoulder, and we proceeded with his order as usual. In the course of this, I discovered that his friends were waiting in the car for the food. He had been assigned to come get the food as practice or something - he didn't like placing his own orders or interacting with strangers, because it often turned awkward, and his friends (and possibly also he) thought he should "just do it" as a way of getting more comfortable.
Anyway, I was able to help him fill the order and make the whole process a little easier, and he seemed so happy to find someone who could converse with him - it really made my day.