I hope you weren’t thinking I was going to reveal some deep secret thing.
My father always told me that you couldn’t share a secret because once you told another person something it wasn’t a secret anymore!
However my father did travel a lot for the government as part of his job and when that involved going to Australia he would have a stop-over in Singapore and bring me back something oriental as a present.
One time he brought back a Japanese puzzle box.
At first sight it looks like an inlaid rectangular prism but if you know the secret you can slide down one of the sides and then slide out the top to reveal an inner space. Having done that you can slide up the opposite side to find an even more secret little drawer that that makes a musical sound (mechanical not electronic!) as you open it. So you would be alerted if anyone else found out the secret and opened it!
But of course I am not going to tell you the secret of how you release the side of the box, because then it wouldn’t be a secret anymore!