1. Sincerely -
Meaning: I find what you’re saying or doing enjoyable or stimulating.
Example: My friend Bob created his own furniture line made out of recycled materials.
Bob: I made this table entirely from recycled car parts.
Me: That’s interesting. (I’ve never seen old car parts used in that way.)
2. Questioningly -
Meaning: I’m not so sure about what you’re saying; I don’t really know what I think about it.
Example: Jeremy thinks it’d be a good idea to invite his ex-girlfriend to his wedding.
Jeremy: I’m still friends with my ex-girlfriend, and I’m hoping that it won’t upset my fiancé to have her at the wedding.
Me: That’s interesting. (I hope that turns out all right for you because my guess is it won’t.)
3. Sarcastically -
Meaning: I really have no interest in what you’re saying; it’s quite boring.
Example: Jim the computer programmer keeps talking to me about computer programming details.
Jim: Actually there are multiple algorisms you can use to get the correct program working here.
Me: That’s interesting. (I am so bored by this conversation.)