Our understanding of atoms has changed a great deal in the last hundred years, but I think starting at the beginning will help you best understand what atoms are. Classical Greek philosophers, notably a man named Democritus, who lived in the 5th century in Greece, were curious about the nature of the world around us. Democritus thought about a very simple situation: If you cut something in half (like a log), the two pieces left can themselves be cut into even smaller pieces, which can then but cut again, and so on down to splinters. But even a splinter could be cut in half. So Democritus asked,"Whats the smallest that something can be cut?" He answer was that, at some very, very small size, you would reach a piece of the object that was 'atomos', Greek for "uncuttable". Fast-forward hundreds and hundreds of years, to the chemists of the 19th century. They finally had the tools and knowledge to investigate the world around us in greater detail, and some of their observations helped bring back Democritus' idea about atoms-- their experiments on gases and liquids seemed to prove that their were incredibly small, indivisible units that made up all matter. That is, if you had a block of iron, and you kept cutting it into pieces, you would (in theory) reach a very small piece which could absolutely not be broken down-- it was the basic building block of iron. This was (and is) a powerful idea that explains all sorts of things about chemistry and the world we live in-- such as how our bodies digest food (since everything is made up of just a limited number of types of atoms (the elements), we can ingest food in one form, break the food down into small groups of atoms, and put the atoms back together as parts of our growing bodies), why only a certain amount of salt will dissolve in water before it starts building up as crystals at the bottom, and millions of other things-- modern chemistry is based on atomic theory. Atoms are almost unbelievably small-- one of your hairs is about a million atoms wide, and a speck of dust you can see might contain trillions of atoms. That's not to say that the idea of atoms discovered in the 19th century was perfect. By the early 20th century, it was well understood that atoms aren't actually indivisible-- they could be broken down into even smaller units called sub-atomic particles (the electron, the proton, and the neutron). But what was indivisible was the chemical nature of the atom. The atom is the smallest piece that remains the same substance. If you have a block of iron, it has certain "iron-like" properties. No matter how small you cut it, the pieces still behave like iron... until you reach the atom. The atom is the smallest piece that still behaves like iron. If you could cut an atom (say, removing a proton from it), the resulting pieces would be smaller atoms-- the removed proton would be an atom of hydrogen, and the remaining piece would be an atom of manganese. Because they are the smallest pieces that retain a unique chemical nature, they are deemed the fundamental building blocks of all matter- the elements. The different chemical properties of the elements are determined by the number and type of their sub-atomic particles.

Atoms are the smallest building blocks of matter. They are like incredibly small LEGO blocks that fit together to form EVERYTHING you see around you. Atoms in every part of your body--your fingers, hair, eyes, teeth...everything. Atoms are in computers, in the walls of buildings, in rocks, in tigers and spiders, in glass windows, in cars, in the air, in plants and trees, in clouds, in the ocean...in EVERYTHING. We can not see them with our eyes, because atoms are so small that one million of them lined up in a row would only be as thick as a human hair.

So, if atoms are in EVERYTHING, why do things look so different from one another?? Ocean water is VERY different from pencil lead, yet they both are made of atoms. Well, the reason things can be so different yet still made up of atoms is because there are more than 100 different kinds of atoms. Each kind of atom is like a different building block. You can put these blocks together in many, many different ways. So, each different thing you see around you is made of a different combination of atoms that are put together in special ways. This is just like LEGO blocks, because starting with the same set of different blocks you can make different shapes--a fort, or a staircase, or a boat--depending on how you put the blocks together. But what are atoms made of? Well, this is actually a hard question. Have you ever taken two magnets and tried to push the same poles of the magnets together? When they come close to one another, you can't see anything between them, but you still feel a force. Atoms are mostly like this invisible force between magnets. Atoms are mostly empty space, but they still have 'force fields' that hold them all together.

Remember that it has taken thousands of years of very, very smart people thinking all of their lives in order to figure out what atoms are. So it is no surprise that it can be a little confusing. Even if its hard for you to understand what atoms are (its still hard for me too sometimes!), keep thinking about it and stay curious! That's the most important thing. If you just stay curious, you can learn more and more about atoms as you grow up.