 Let's think of some examples of the sort of pressure at use. One example would be a spaceship. If you're in a spaceship, you've got your astronauts inside, and they hopefully got air to breathe, but there's no air outside. So what that means is there's going to be a fairly substantial pressure pushing outwards. So your windows, for example, have to be bolted in. If they weren't, the pressure would push them out. So let's say you had a window that was 10 centimeters, 0.1 meter by 0.1 meter. And let's say the pressure inside your space station was the same as at the Earth's surface, 100,000 pascals. There'd be an outward force on your window, which is equal to the area. The force is equal to the area times the pressure. So that would be 0.1 times 0.1 is the area times the pressure, 100,000 pascals, roughly speaking, which comes out as about 1,000 newtons. So that's how strong the bolts would have to be to hold your window in. Another example would be a submarine under the sea. When you're underwater, the pressure can be much, much higher than at the Earth's surface. Say you're at the bottom of the Mariana's trench, the pressure could be thousands or even millions of times higher. So that means if you want to be inside it and not crush flat, you have to have a very strong skin because the air pressure inside would be, say, the normal pressure at the surface, but the pressure outside would be much bigger. You're being hit by so many fast-moving water molecules that it will crush your submarine unless it's very, very strong. This is actually the problem for most real submarines. They dive too deep, the pressure outside gets so big, it crushes them and then, therefore, they collapse. That's why a normal submarine can't go to the bottom of the Mariana's trench. You need a special extra-thick ward submarine to go down that deep because the pressure is so great. Another example of pressure is drinking. Let's say you have a cup of some soft drink and you want to suck it up. So you apply your mouth to the top here and you suck. But you can't really suck things up. What's actually happening is you drop the pressure inside, but the pressure outside of the atmosphere is still very big and, therefore, that pressure pushes the fluid up to your mouth. So you're not really sucking the water up. The atmospheric pressure is pushing it. If you tried this in a vacuum, I wouldn't recommend it, and you sucked, nothing would happen. The water would just sit there. There'd be a vacuum inside here and a vacuum out there and the gas wouldn't move at all. Another example is suction caps. Often, let's say you want to pick up some big sheet of glass in a factory. What you would do is you'd have cups on top, like rubber cups, and you suck the air out of them and then they could be attached to ropes and they can pull the glass up. And that is, in fact, how glass is normally handled. What's happening here, you're not really pushing stuff up. You've got your piece of glass and you've got the suction cap on top. And the pressure inside is very low. What's actually happening is the pressure underneath is pushing up. So, in fact, when you put a suction cap and pull a piece of glass or a piece of metal or something up, you're not actually pulling it up. What's happening is the atmospheric pressure is pulling it up. All you are doing is removing some of the pressure at the top so it's no longer perfectly balanced. So that's pressure. Pressure is the force of billions of atoms hitting you at all points. The force applied to some area is equal to the pressure times the area and the pressure applies in all directions.