It's polite to tip 20% for good service.
Ted orders a large amount of salad from a restaurant, his waiter was very good, and he receives a bill from the waiter.
Ted wants to add a tip to the bill for the waiter.
Ted adds a 20% tip for his appreciation of the waiter.
Ted adds a 2% tip and waits for his debit card back.
The waiter thanks Ted and lets Ted know he can come back whenever.
The waiter gives Ted a foul look as she returns his debit card.