Question

why do you find it hard to push a glass tumbler down into the water?

Answer 1

the air present in the glass tumbler exerts pressure on the water when the tumbler is pushed down into water vertically and so we find it hard to push the glass

Answer 2

It is hard to push the empty upside down mug inside the water as it has air present inside its cavity which prevents the water from entering inside the mug.But when we tilt that mug which is still completely immersed in the water by some angle then water enters inside it accompanying the formation of bubbles.....even thogh buoyant force acting would be the same in all orientations it happens