Mad Science Camp: Poly Density Kit

I ordered the Poly Density Kit back in March for Science Camp from Educational Innovations.  Paul is doing Camp activities all summer.  The density bottle is one of my favorite discrepant event; the bottle makes kids thinks.  Why do the blue beads sink and the white beads float on top?  Even teens who assemble the bottle from the kit struggle with understanding how the beads float. Kids will shake the bottle and watch the beads over and over.   Here are pix.