What are atoms made of? How do we know?

All matter is made of atoms.  Whether it's carbon, hydrogen, oxygen, sodium, or some other element, we are composed of atoms, and everything else in the universe is also composed of atoms.  But this begs the question - so what are atoms made of?  Over the next two weeks, we will be diving into what atoms are composed of, and how scientists were able to figure this out. 

So what do you know so far?  What are atoms composed of?  How do you think scientists figured this out?  Please comment on the blog post below.