Colour Changing

On a table are a number of large glass vessels. If you pour purple liquid from the first vessel into the second, empty vessel, the liquid suddenly changes colour. By repeating the process for the third and fourth vessel, the liquid again changes colour each time.

How does it work?
In the second, third, and fourth vessel is a small quantity of a particular salt. The purple indicator solution poured from the first vessel reacts and is discoloured by the slight acidity, turning yellow when reacting with the small amount of FeCl (III). In the third and fourth vessels are other salts, which form iron complexes and have different characteristic colours.