This a classic kitchen chemistry experiment.
Fill an empty cup with a paper towel and gravity and make a rainbow of colors!
Have you ever wondered why paper towels are so absorbent?
Paper towels are made of many small fibers that have gaps between them which means they use capillary action to absorb liquids into these fibers.
Surface tension fuels this process. The surface tension of water is caused by cohesion, which is the fancy science term for water molecules being attracted to one another. The reason water forms beads instead of spreading out is another example of surface tension, and this also allows the water to get sucked up and absorbed into the gaps of materials (in this case, the paper towel!). Absorption into the paper towel is given a little help from adhesion, which is the attraction between different types of molecules.
Another factor that makes this experiment possible is the fact that paper towels are made of cellulose which is polar - meaning it has a negative charge on one end and a positive charge on the other end.
Guess what else are polar!?! Water molecules! The opposite electric charges are attracted to one another, and thus the water molecules are attracted to the cellulose fibers.
Scissors (if you need to trim paper towels to fit cups)
Optional: cookie sheet or tray to contain any spills
Assemble the cups in a circle or a line.
Add water and a few drops of food coloring to every other cup as pictured below. I used the primary colors of RED, YELLOW, and BLUE so that I can make a rainbow and observe color mixing in action!
I used an even number of cups, but you can make this in a simple line with an odd number of cups, too.
Fold the paper towel lengthwise (like a hotdog bun) and bend in a V shape so it can stand in two cups simultaneously.
Capillary action begins as soon as you place the paper towels into the colored water. I suggest pressing the wet towels to the side of the cup. Wait 30-60 minutes (if your cups are short) for the full effect!
Color mixing of red and blue:
Water walking and color-mixing success: a rainbow of colors after the process finished!