We have had one of these around the house for years. Our grandchildren, of all ages, love to watch the water swirl from the top bottle to the bottom bottle. Our 2 ½ year old grandson would bring it to one of us, to have us swirl it, over and over again. The first time I saw this principle used in life was when a Marine swirled a bottle of wine into a punch bowl, at a military wedding. It was way cool then, just as cool now. Here is How to Make a Vortex in a Bottle.
2 straight walled, 2 liter bottles with caps
5 minute epoxy (about $4.00 at craft stores)
Small paper cup
½ drill bit
Sand paper, medium grit
Food coloring (your choice of color)