This simulation shows how energy tends to spread out and flatten, even though the total is conserved.
Even with this small 20 x 20 grid, the simulation is having to update about 100,000 times per second, so you're asking quite a lot of your processor.
For this reason the mouse clicks can be a little sluggish, and you may need to long press on the grid.
You can press multiple times wherever you like, and see the total thermal energy rising up on the graph.
You may notice that the energy sometime seems to flow uphill. This is because the energy per element is calculated at a greater resolution than the colour scale allows. You still get the idea of the energy flattening out to a new higher level each time you add energy.