Don't overthink it. What may come easy to you, and may not appear "smart" to you, may be to others. If someone pays you a compliment, feel good about your self, and acknowledge it. If you think there is nothing to feel good because "it is not true as it isn't very impressive", realise you are being too literal - you should feel good about helping someone with your skills, or teaching someone something they didn't know about, and telling you "You are smart" is their way of appreciating it and thanking you for your help.
Compliments are all about appreciating one another, and it’s just as important to learn how to graciously accept a compliment, as learning how to give a good one. When you deny a compliment, you may be giving the impression that someone else’s perspective is wrong, and you may make them feel bad for trying to say something nice to you. (Ref. 1)
I. In a formal setting (school, work, strangers etc.) acknowledge the compliment by saying:
- Thank you, it is very kind of you to say that. (or)
- I appreciate you saying so.
II. In an informal setting (friends or family), you could:
Joke about it:
- "Finally, you acknowledge it!"
- "Yeah, I am a genius!"
- "Are you saying I am smarter than you too?" etc.
... and then acknowledge it formally (see I).
Note that whether others find the joke humourous depends a lot on how you deliver it. It helps to remember that the success of the joke depends on the non-verbal cues too - your body language, your facial expression, your tone etc. - when you say it. And it also depends on how well you know the other person and understand what kind of sense of humour they have and what makes them laugh. If somebody doesn't get the joke, it is ok - just say, "I was joking. Thank you very much for your compliment. It felt nice to hear it."
Acknowledge it formally (see I) and add a clarification (if you want to).
- [Thanks ..] The truth is, I am good at it because I spent a lot of time practicing / learning it.
Acknowledge it formally (see I) and offer to help
- ... and since you are my dear / good friend or such a nice / kind/ person, let me know anytime when you want my help on this.
(Or use a combination of 1, 2 or 3).
Offering compliments are sometimes also an attempt to convey to you that the other person likes you and would like to get to know you better. (Ref. 2)
(Note: Customize the language according to your own personality and culture).
References:
Why Giving and Receiving Compliments Feel Great
The Psychology Behind Compliments