Rainbow in a Bottle

Materials you'll need

• Small cups
• Water
• Red, yellow, green, and blue liquid food coloring
• Salt
• Measuring scoop
• Clear glass bottle
• Straw

 

Step-by-step tutorial

Step 1: Fill 4 small cups with water, then add a different food coloring to each one — red, yellow, green, and blue.

Step 2: Stir 1/2 teaspoon of salt into the yellow cup, 1 teaspoon into the green cup, and 1 1/2 teaspoons into the blue cup until it dissolves. Leave the red cup with no salt at all.

Step 3: Pour the red water straight into a clear glass bottle to start your rainbow.

Step 4: Push a straw all the way down to the bottom of the bottle. Slowly pour the yellow water through the straw so it settles underneath the red, then repeat with the green, then the blue.

Step 5: Pull out the straw, twist on the cap, and check out your rainbow — blue on the bottom, then green, yellow, and red on top, all staying in their own separate stripes.

Learn more

This colorful stack all comes down to density — how much mass is packed into a certain amount of space. Dissolving salt in water adds mass without adding much size, so saltier water is denser (heavier for its size) than plain water. Blue has the most salt, so it's the densest and sinks all the way to the bottom, while the plain red water has no salt and floats to the very top. Because each color has a slightly different density, they settle into neat, separate stripes instead of swirling together.