You display the wooden box. There is a small window in the top which allows a view of the empty interior.
A borrowed dollar bill (or any small object) and the box and put behind your back for just a few seconds.
You bring the box back into view and the bill is now inside!
The spectator may examine the box & will not find the secret. They cannot open the box.
Again place the box behind your back and in an instant, the bill is free from the box.
This effect can just as easily be performed under a handkerchief in front of the spectators.