Everyone knows what the word 'tip' means in the context of a receipt or a bill, but some people still find it hilarious to pretend that they don't understand. Here's a memo for all those people: Service staff have come across every single one of your jokes, and they've ever found them funny.
Nobody should look directly at the sun, so as advice for life this tip works. As a way of thanking whoever served them for their dining experience, it's a cheap and nasty thing to do.