As long as we've watched birds in the skies, a lot of people have wanted to fly. For a long time, this wasn't even a possibility for people! And than... along came the inventors. And until about a century ago, there wasn't a power source strong enough to sustain flight without using the natural air currents. Wind is how the Earth distributes temperature around the planet. The atmosphere is a "sea of air" that is in constant motion. Sometimes it's hot and dry, sometimes cold and wet, but always fluctuating in temperature and pressure. When you look at a balloon (imagine it hasn't been inflated yet), the same amount of air pressure is both inside and outside of the balloon. Now imagine blowing up a balloon. What did you do? You put more air inside the balloon, so how now there's more air showed into a smaller space than was there a minute ago. You have increased the pressure. The balloon can hold its round shape because there's more air inside than before it was inflated. Imagine putting a straw into your lemonade. How do you get the lemonade to move UP the straw, up a higher level than the top surface of the lemonade?  You reduce the amount of air in the straw (this is opposite to what you did with the balloon), and now the lemonade zips up the straw, seeming to defy gravity! You lowered the pressure in the straw, and the lemonade went up the straw. It's easy to make the assumption that lower pressure pulls things, like the lower pressure int he straw pulled up the lemonade. Not so fast! That actually happened is that higher pressure pushed the lemonade up the straw. Higher pressure always pushes. Lower pressure does not "pull". The higher pressure inside a balloon pushes outward and keeps the balloon in a round shape when inflated. When the pressure in the straw was lowered, it was lower than the surrounding air, which is now as a "higher" pressure and pushes the lemonade up the straw. By "higher" I mean: "higher when you compare it with the pressure in the straw". Please login or register to read the rest of this content.