Question

the piece of a post card does not fall off the glass though the glass is full of water and its whole weight is exerting pressure on the card why​

Answer 1

Explanation:

The reason this experiment works is because of air pressure! Air pressure is the weight of a column of air pushing down on an area. ... Because of the air pressure pushing up on the card, the card will stay on the glass and the water will not spill out.