Question

why Cotton wool in the tumbler do not wet when the the tumbler is immersed in water upside down

Answer 1

The cotton wool was dry when you pushed the glass into the water, because there was air in the glass and water could not go in. When you tilt the glass, air escapes as bubbles and water enters the glass. This shows that air occupies space. Fill a glass completely with water and cover it with a piece of stiff cardboard.