Chemistry experiment - Coloured flask.
When a very dilute solution of permanganate is slowly reduced to colloidally dispersed manganese (IV) oxide, then a beautiful range of colors is traversed, starting from purple/violet and ending in yellow/brown. It is amazing to see the solution change color. For this reason, in the past, permanganate sometimes was called 'chemical chameleon'. Only at a later time, it was understood that this remarkable phenomenon just is the result of change of oxidation state, and the mixing of colors of different compounds. From a chemical point of view, this experiment is not that special, but it still makes a nice display, maybe even more so, now it is well understood.