Hi Sinem, Something is on your mind if you are thinking of something repeatedly consciously or subconsciously. Example: Although she has left me, she will always be on my mind. Something puzzles you if you are not able to comprehend it, or if it does not conform to logic. Example: His erratic behaviour really puzzles me. Hope this helps. Cheers, Lance

Great point Jan de Boer!

Something's been on my mind. - This can mean that you are pondering anything at all. It could be a person, an experience etc... There's something that puzzles me. - This means that there is something you are thinking of that is not clear. It is a "problem".

Jan, that's a good explanation...the two can mean the same thing, but the first one has more meanings to it depending on the context.

In my humble non native speaker ears there is a nuance. There is something on my mind. That does not have to be a puzzle, a logical problem. If somebody is sad for example you can say: Is there something on your mind? There is something that puzzles me would slightly more refer to a logical problem. Puzzles refers to something that seems to be in paradox with logic.