The demonstrator takes a stick of rhubarb, taking a bite out of it is a nice touch, and uses it to stir a solution of potassium VII manganate, which goes clear.
Rhubarb contains a chemical, oxalic acid, that reacts with the purple permanganate ions. Colourless manganese ions are formed.
This can be extended into an investigation (See Rhubarb Rhubarb!)
