Click/tap on a cell (a square in the grid below) to select it and add heat to that cell. (Selected cells have a red borders.) You can heat as many or as few of the cells as you like.
Each heat interval, after the heat is added, the heat spreads from each cell to its neighbor cells. Heat will be lost off the grid on all sides.
Choose the grid size; resizing will start over.
Choose the heat interval speed. How many times does the screen update per second (1-20)?