CRAZY interactive website that shows relative sizes of things... From a coffee bean to a grain of salt to a human skin cell to an E. coli bacteria to an HIV virus to a carbon atom with a bunch of others thrown in for comparison. WOW!
Mitochondria and chloroplasts, organelles which synthesize energy in cells, were once single celled organisms. These cells were engulfed by larger ones and eventually they could no longer live on their own. The is called the endosymbiotic theory. Click the link for more information!