when a girl says she can't stop thinking about you?
Say it back if that's what you feel too. If not, just smile & say thank you :)
Say the same back (if its true) and if not just tell her its appreciated. If you don't feel the same then tell her.
depends how that makes you feel and how you feel about her
let her know that you appreciate it. don't say um or okay or something like that, that's disrespectful and makes her feel awkward. but if you don't like her, make sure you don't lead her on, you don't want to hurt anyone