Even preschool kids know that when you mix red and blue, you get purple. So, why are the two jars of colored water shown here so uncooperative? Try this magic trick, cleverly disguised as a science experiment, to surprise the know-it-alls in your family.

MATERIALS

2 identical baby food jars
Kitchen faucet
Red and blue food coloring
Index card or playing card

DIRECTIONS: Working in the sink or some other waterproof area, fill one jar with hot tap water and add a drop of red food coloring. Fill a second jar with cold water and add a drop of blue food coloring. Slowly add more hot water to the red jar, until the water seems to bulge over the rim. Lay an index card on top of the red jar and tap gently. In a swift motion, turn the jar over. The card will prevent the water from spilling out. Place this upside-down red jar on top of the blue jar, as shown. Have an assistant hold both jars steady as you pull the card out. Although a thin purple line forms where the colors meet, the red and blue water will not mix.

WHY IT WORKS: Because hot water, like the hot air in a balloon, tends to rise. That's because it's less dense than cold water (the molecules in the hot water move faster and stay farther apart). So, the cold water stays in its bottom jar, while the less dense hot water floats above it. Now, what do you think would happen if you put the cold blue water on top? See if you're right!