How cards can stick to a glass if upside down with water in it
In: Chemistry
The surface tension of the water helps adhere the card to the rim of the glass. Simultaneously, the fluid in the glass is (effectively) incompressible, so the volume of the water doesn’t change, and the card stays there. If the fluid changed volume — the way gases do — it could expand and let the card fall away. But it doesn’t, so the surface tension holds the card, and the lack of air entering the glass means it stays there.
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