Chemistry
Chemistry is the science (and the art) of making new compounds out of natures builing blocks (the elements), and of purifying existing materials in order to extract useful components.
Chemistry, while very simple in its basic nature is a fairly complicated branch of science. The amount of chemical compounds is theoretically pretty much unlimited, the trick is to make those compounds that have desirable properties (as opposed to say mountains of brown goop).
Chemistry has a very rigid naming scheme for the basic elements, their interactions and the resulting compounds.
A simple chemical reaction would look like this:
2H2O + Energy -> 2H2 + O2
In human speech that reads as 'two molecules of water plus some energy result in two molecules of hydrogen and one molecule of oxygen'.
The effect of this in practical terms is that we have broken up water by adding energy (for instance electricity) to create hydrogen gas and oxygen gas !
Now we can do the reverse:
2H2 + O2 -> 2H20 + Energy
Which reads as two hydrogen molecules + one oxygen molecule result in two molecules of water and energy.
This reaction is the basis for the 'fuel cell', a device that uses hydrogen in tanks and oxygen available in the air to produce electricity. It is being tested to power electric vehicles.
Most chemical reactions and their corresponding formulas are a whole lot more complicated than that !
Allmost all of the interactions in the world that are not simply collisions are chemical in nature. Every time someone cooks a dinner or makes a cup of coffee that is applied chemistry.
Chemistry has grown out into a huge branch of industry, good for untold billions of dollars of turnover the whole world over. Without chemistry no plastics, no gasoline, no food preservation and almost no medicines other than those found in nature.
In fact, it is very hard to think of getting through an hour of the day without some kind of chemical activity, our bodies are nothing but very complicated chemical (and electrical, but that's another story) themselves.