When introducing this cool little item I like to present a story ...
I keep something small inside of it, maybe a lucky coin or a ring, a marble from my childhood, something that will fit in the box and has some significance.
When I bring out the box, I say ...
"So I have this black box, and inside it is a smaller red box where I keep my (whatever), but sometimes ... (I now put the item inside the black box and close it) ...I prefer to keep the item in the black box ... (I put the black box inside the red box, close it, and just wait for a reaction. If they were paying attention there will be a reaction!)
These boxes are cool! A physical impossibility! A neat little pocket paradox to carry around and show the curious!
The Effect: You show a small red box. Open it, and from inside you remove a small black box.
Now for the cool part!
You close the red box and place it inside the black box!
Wha?! That's right!
And remember, the black box came out from inside the red box, now the red box is inside the black box!