How Do You Tell Someone That They Are Sexy?

Well, there is always a timing for things. I have this boy in my class who is really sweet. When I came in to class I said 'it's really hot in here' and he said 'that's because you just walked in, sexy'
It flatters girls for boys to stay stuff like that, no matter how silly.
(Be unexpected!!! For my birthday, he sang me Hello by Lionel Richie, a love song!!)