I recently did a classroom presentation with candy experiments, and finished up with the Skittles density rainbow for my grand finale. It worked pretty well, thanks to these preparations:
--I counted out the Skittles ahead of time and put them in cups: one set of
1/5/10/15/20 starting with 20 reds, and another set starting with 20 purples. That way, I had a backup set.
--I stacked the cups for each color set and carried the Skittles in the cups
--At the school, I measured out the water (as hot as I could get out of the tap), and got my Skittles dissolving before the presentation started
--I asked the teacher to stir my cups occasionally during my presentation
By the end, my red and orange hadn't quite dissolved, but I went ahead and started pouring. Red and orange blended together a little bit, but the green and purple stripes were very distinct. Success!
One thing I'll remember: pouring the rainbow takes a little while, so it's best to talk to the kids while you pour. The trouble is, you can't see the kids to call on them. Next time I'll invite the teacher to call on kids while my eyes are fixed on the rainbow.