Understanding Pressure

In order to understand Multi-Stage Flash Distillation one needs to understand Boiling Points of Water. In order to understand boiling points of water one needs to understand Air Pressure.

Pressure seems like an easy thing to intuitively understand, but it might not be as simple as previously thought. We’ve all swam down to the bottom of the deep end of a pool and the pressure of the water hurt our ears. This is a simple example of our common understanding of pressure.
When we start to measure pressure things get a little tricky. I can weigh objects with a scale, but I can’t take that same scale and measure pressure even if I feel it like the bottom of the deep end. If I put the scale in the low end of the pool to weigh water it won’t measure anything nor if I put the scale in the deep end would it change. We know the deep end of the pool has more pressure but how can we measure it? There are a few ways to measure pressure, but thats not needed to understand pressure. The idea of measuring pressure helps to understand what it is and how its working.
How much does water weigh? We can take a 1 liter bottle and put it on a scale and you might tell me that 1 liter of water weighs 1 kilogram (2.205 lbs.) But if you were in a pool, underwater, how could you tell me how much water weighed if you couldn’t get out of the water? Also If you were in the deepest parts of the ocean and filled that same bottle with water it would probably explode on the way back up b/c even at 10 meters underwater it would already be filled with 2 liters of water. How is that possible? Pressure. 10 meters (33’) underwater is double the pressure of being out of the water (or 2 atmospheres). All this makes the question how much does water weigh difficult to answer. This is all still avoiding temperatures effect on all this. (Water expands when frozen, for example, so 1 liter bottle of frozen water would be less than warm).
Air has similar problems. If I could hand you a box of air, and asked you does it weigh anything, I imagine most people would say no it doesn’t. B/c we use common sense and by comparison to things we consider weighing something the air doesn’t weigh anything. But you would agree that there are molecules of “air” in there (nitrogen, oxygen, water vapor, argon, and carbon dioxide - lets call it “air”). If I had a box of “outer space” and a box of air, I think we’d all agree that air has far more molecules in it than outer space. So air does weigh something, just very little. It’s also hard to measure since everything we do is surrounded by it. Which is why it’s easier to describe air in pressure than weight. Similarly to the deep end in a pool, there is air pressure from the 30km-ish (15 miles-ish) of air above us.  
Air pressure can be simplified like this. If we had boxes of air, as you stack them one on top of one another, the bottom boxes would start to squash from the weight of the boxes above it. The more the box is squashed the more pressure it has. This is why higher altitudes have lower pressure, there are fewer boxes of air above them.