WARNING! SKIP THIS POST IF YOU DON'T WANT TO KNOW HOW THE TRICK WORKS!
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The store owner is on in the trick. There is a separate, loose pane on the inside of the window. When the paper goes up, the interior pane slides up. The reason he asks the man who puts his hand on the paper not to press too hard is because if he does, he will feel there is no glass. The pane slides back down when the trick is over.
By the way, glass actually is a very slow-moving liquid.
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