Instantly change a big dice into 8 tiny dice, simply by a shake of the box.
The big dice is hollow inside, with one side open.
Put the 8 tiny dice into th big hollow dice and then carefully put the big dice into the box, making sure the open face is pointing downwards.
Close the box Close the box with the cover. Show the big dice to the audience and ask one of them how many dice are in the box. They will of course say just one.
Now just shake the box upwards and quickly. the big dice is magnetic and the cover has a metal inside, so the shell of the big dice will stick to the cover, but will not be visible to the audience. The 8 small dice will fall back down into the empty box.