Subject: natural indicator solutions

While messing around looking for natural dyestuffs, we discovered that the juice 
from blueberries is a natural indicator -- when you add acid it turns quite red 
and when you add base it turns dusty blue. Tea turns from gold to red.   Does 
anyone know of other easily collectable natural substances that do the same 
thing?  Also, we would love to understand HOW indicators work. (We have a 
collection of kids from elementary age through high school.)
Thanks. 

Re: natural indicator solutions