The last thing you want to do while working in a restaurant is watch people eat. Sooner or later there will be someone that will disgust you by eating with their mouth open.
Back in the 60s you could smoke in all restaurants. People would put out their cigarettes in the remains of food. They sometimes left cigarettes smoldering in the ashtrays.
One day there were two teenage boys sitting at the counter and giggling all the time they were eating. It was after they left that we found out that they had sprayed ketchup and mustard all over the front of the counter. Being the low man on the totem-pole, I was the person that had to clean up their mess.
See that picture? It is a picture of gum on a sidewalk. This happens to be out in front of the local Walmart but every business has those spots by their front doorway. Gum is very hard to get rid of because it semi-liquid. One method is to use high pressure steam. They also sell a spray that freezes the gum and then you can use a scraper to pop it off the concrete. Either method takes a lot of time. Please, don’t spit your gum out on a sidewalk.
People also left their gum under the lip of our counter and also under the seats of the stool. Again it was hard cleaning off the sticky gum mess. After all these years I still don’t like gum.