Tried this demonstration at home before I try it in the classroom. It shows how hot and cold water can both mix or not mix together. I used blue food coloring for cold water and yellow food coloring for hot water (didn’t have red handy at home, but will use red at school). It also demonstrates a great surface tension trick with the upside down jar of water and a playing card. I use playing cards b/c the waxy surface works best for either demo. (I used old glass jelly jars, we go through a lot of jelly at my house. I run them in the dishwasher and save them for science experiments.)
For this demo, I am not going to tell the kids that I am using cold and hot water. Want to see if they can figure it out 
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When the cold blue water is on the top, and the hot yellow water in on the bottom, as soon as you pull the card away, they mix and the water turns green in both jars. (purple if you use red). When the cold water is on the bottom, and the hot water is on the top, when you take the card away, it does NOT mix like before. there may be a small zone of mixing where the two meet. Great discussion about density and the effects of temperature on the movement of the water particles.
It is a pretty cool trick and I think the kids will love it when they see it, only b/c it is so unexpected for them .
For more information, check out this website: http://www.exploratorium.edu/science_explorer/watertrick.html
