When the handkerchief is loose, the water can pour through the gaps in the fabric. But when it is pulled tight, the water molecules are able to form a single membrane across the material due to surface tension.
At the same time, the air surrounding the glass is pressing on this membrane, and as the air pressure surrounding the glass is greater than the pressure inside, this helps hold the water inside the glass.
The key to this trick is to pull the hanky really tight – think of those old-fashioned canvas tents that Scout and Guide groups used to use. The canvas was pulled really tight to keep out the rain, but you couldn’t touch the sides of the tent otherwise water would come through the canvas and you would be set for a very soggy night’s sleep.
It’s the same with this trick – due to surface tension, the water forms a membrane across the hanky, but if you poke the hanky with your finger then you break this surface tension and the water comes through the cloth.
Find out how to perform this and many other tricks with Physics to Go.
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