Drinking through a straw
When you drink through a straw from an open glass of water, air pressure allow the water to travel up the straw. By sucking on the straw, you are reducing the air pressure inside your mouth.While sucking in the straw, the air pressure in your mouth is less than the air pressure outside of the straw - in the room and in the glass. The outer air pressure pushes down on the water in the glass and this forces the water up the straw.
Can you predict what will happen if you put an airtight over the glass of water?
No matter how hard you suck, the water will not go up the straw. Why? Well, when air pressure on the water is blocked, there is no air pressure to help push the water up your straw. The air can't get to the water to push on it, so it doesn't go up the straw.
Unclogging the sink with a plunger
A plunger is made of a long stick and a bell-shaped, sturdy rubber cup on one end. The rubber cup of the plunger is set onto the draining hole of the sink to create a seal.The plunger stick is then pushed down, inverting the rubber plunger cap and forcing the air that was in the plunger cap into the pipe.
The plunger is then pulled back, sucking air and water up with it, causing a vacuum. This abrupt force of air and water pressure helps loosen a clog in the pipes and gets the sink water draining again.
It may take several repeated motions of pushing and pulling back on the plunger to loosen the clog.
(Natasha Aleia)