REALLY SHOULD MAKE A VIDEO OF THIS NOT HARD!
Domestic hot-water and heating systems.
In Liquids and gases the major method of transfer of heat is Convection. This is where the molecules move and deliver their heat energy some place else. They heat up, the molecules gain kinetic energy and the vibrate more vigorously, this makes them bigger but the mass has not changed, therefore their density goes down, therefore they float towards the top.
only add the dye after the liquid has been heated for a while!
SEI to be explored