Swapping Spit

This thought crossed my mind at lunch today. How does the the waiter/waitress know whose cup belongs to whom when they take it to the kitchen for a refill? It's not rocket science if there are two cups. Lefty belongs to Steve, righty belongs to other. What if there are 3 cups or 4 cups? Kinda makes you wonder if the straw you're sucking from really has your own spit on it, doesn't it?