Convection tube potassium permanganate
Fill the convection tube with cold water. Drop a single, very small, crystal of potassium manganate into the tube so that it falls to the bottom, leaving little colour.
Heat it gently with a Bunsen flame at the bottom of the tube to see the streamers of dye begin to circulate. As soon as the tube is felt to be warmed by the hot water, stop heating.