This article from the History Channel's website goes into depth about the conditions found in the trenches during World War 1.
Later we learn more about the particles that matter is made from. All matter is made from atoms. Sometimes these atoms bond together to make molecules. In time we will learn about the difference between say water molecules and hydrogen and oxygen atoms. For now you can watch this video to find out what an atom actually is.
A weapon from WWI that originally had a cast iron case.
This periodic table has videos attached to every element. Go explore!