You should say thank you when someone makes you something.
It was Kate's birthday and her best friend baked her a cake.
Kate wants to let her friend know she ate the cake.
Kate calls her friend up and tells her thank you for making the cake and that she ate the whole thing.
Kate brought the cake container back to her friend without saying a word.
Kate's friend is thrilled that her friend liked the cakes.
Kate's friend never goes through the trouble of making her a cake again.