Floating....

So, here is the first answer:
http://www.pa.msu.edu/sciencet/ask_st/031192.html

Generally a good answer, but even my five year old will hear the word atom and his eyes will glaze over and he'll want to go play Ben10 or watch oldish Batman cartoon movies..

so, the second hit gave me:
http://www.newton.dep.anl.gov/askasci/phy00/phy00291.htm

I like this one! :)
Enjoy!