Every year my grandparents would host a massive Christmas dinner for all their friends. One year as everyone sat down to enjoy their feast my grandfather was no where to be seen. " Where is Otto?" asked the guests as they helped themselves to the steaming dishes they were being served. "I have no idea" said my grandmother looking distinctly irritated. 

As the dinner progressed through 5 courses there was still no sign of him and his place at the table remained empty.

As the cheese arrived, my grandmother stood up in exasperation and apologised for his absence, exclaiming that she had no idea what had happened to him.

At the same moment my grandfathers voice interjected. "I have been here since the beginning of the dinner, but knew that none of you would look up to see who was serving you"

Every since then I have always made a point of looking up at the waiter or waitress serving me and saying 'thank you' when they deliver or serve the food. It was a good lesson.