WHAT TO DO :-
Once you have everything in place, fill up the glass with water right upto its brim. Carefully place the glossy side of the postcard down on the rim of the glass. Keep the palm of your hand placed on the card and turn the glass upside down.
WHAT HAPPENS :-
The card remains attached to the rim of the glass and does not allow the water to flow out.
WHY DOES THIS HAPPEN :-
This happens because the air pressure exerted on the card from underneath is greater that the weight of the water inside the glass. This is why the card manages to hold up the water not letting it spill out.
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