“Color Trick,” Friend, Mar. 1995, 23
Cut a leaf of red cabbage into thin slices. Put them into a small bowl, and pour boiling water on them. After they have cooled for half an hour, pour the violet liquid into a glass. Now you’re ready for the trick. Put three glasses on a table, one containing water; the second, white vinegar; the third, soda diluted in water. Pour a little bit of the violet solution into each glass. The liquid in the first will remain violet; in the second, it will become red; in the third, green!
The chemical property of cabbage makes its juice turn red in an acid, and green in an alkaline solution. In a neutral one like water, it doesn’t change.