Using clay, make a base for the straw. Stick the straw into clay to keep it standing straight. Now, using the eye dropper, put drops of the different colored water into the straw. The different colors float on top of each other in the straw. That's because the more salt there is in the water, the denser the water is and the lower it will be in the straw.
Density is one thing that makes things float.
So, the color that floats at the top of the straw has the least amount of salt in it and is the least dense. This is kind of complicated, so here's the science scoop again. The color with the most salt in it is the most dense, so it stays at the bottom. The color with a medium amount of salt in it is medium dense, and the color with no salt in it is the least dense. I liked it, and most of all it was fun and neat, to see everything float on top of each other.