Thionin—The Two-Faced Solution

A large beaker contains a bright purple solution. The beaker is placed on an overhead projector that is half-covered with aluminum foil—half of the purple solution is sitting on the piece of aluminum foil, the other half is sitting directly on the overhead stage. Switch on the overhead lamp and, in seconds, the solution on the side of the beaker exposed to light turns colorless, while the unexposed side remains purple. The result is sharp and stunning—a two-faced solution! The solution appears to be divided by an invisible line running vertically through the solution. Amazing enough, but switch off the overhead light and the entire process can be reversed.