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Take a head of red cabbage and slice it into thin strips. Let it simmer for 30 minutes in enough water to cover the strips. Strain the juice through a colander and store it in another pot. Let it cool. Explain that the red cabbage juice is called an "indicator." This means that it will help you figure out if a substance is an acid or a base. Take some of the juice and pour it into several clear cups. Put the rest to the side for now. It doesn't matter how much you pour in each cup. A few ounces will be fine, and should leave you enough experiment with several substances. Use as many cups as you have substances to test. For instance, if you want to test milk, tomato juice, and soy sauce, use three cups. Take a spoonful of baking soda and pour it into one of the glasses. Have a kid stir until the soda begins to dissolve. The solution will turn from red to blue or purplish. Explain the indicator solution changes this color because baking soda is a base. Take some ordinary white vinegar and pour it into the same glass as the baking soda. Ask a kid to stir the solution. It will turn red again before your eyes! Explain this is because the acidic vinegar changes the pH of the solution by neutralizing the base (baking soda). Practice stirring different substances into cups of the solution. Drinks like cola, lemon juice, or milk work well. Before you try each substance, ask the kids if they think the solution will turn bluish (meaning it's a base) or a deeper red (meaning it's an acid). To help the kids decide, ask them to think about whether or not the substance tastes sour (acid) or bitter (base).

Summary:
Make some red cabbage juice. Pour some of the juice into clear cups. Add baking soda to the solution. Pour vinegar into the solution. Try adding different substances to the indicator solution.