(Sarah of the Clue Crew gives the clue.) We've rubbed the pigment of a radish on these cards; lemon juice turns the pigment orange, indicating that it's an acid; milk of magnesia turns it blue, indicating that it's this
We've rubbed the 
pigment of a radish on these cards; ... lemon juice turns the pigment orange, 
indicating that it's an acid; ... milk of magnesia turns it blue, indicating that it's this | 
a base.