Indicators work by turning a distinctive color in the presence of an acid or a base.
You can make your own indicator from red cabbage. You can also make indicators from the juice of elderberries, blackberries, or cherries. Boil a pint of water in a saucepan. Add the red cabbage carefully to the boiling water and take the saucepan off the heat. Let it stand for 30 minutes until it is completely cool.