You are not going to like my answer: "Because it is their nature."
Yeah. Told you. It is best understood by studying the Lorentz Force and working your way from there. I like this site: http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html (click on the bubble to expand) but, there's always Wikipedia. Now Brian Greene would have me say "Because it is their nature in this universe." T
