These days there's something wrong with my teeth.In the evening I can't fall asleep, so I had to see the dentist.Luckily the dentist is my colleague's friend, so he's very friendly and kind to me .He looked over me very carefully.He told me one of my back teeth was bad, it needed to be mended and healed.He operated on my bad tooth at once.He gave me a needle, but I didn't feel painful at all.After the operation, about half an hour later I feel a little painful ,and there was a lump on my right face.The dentist asked me to go to the hospital for the further healing the day after tomorrow. When I asked him how much I should pay for him, he said it's free. At first I can't belive this, especially in this money-oriented society, but he said this operation was easy for him,he was very happy to help me. I think he is a good doctor who has outstanding skills and excellent morals. I want to thank him, but I don't know how to do it. Can you give me some advice?