You can click individual cells to turn them on and off, or you can click and drag to turn on areas of cells. You can click and drag once the process has started. The speed slows down while the mouse is clicked, to make it possible to create a track of 'live' cells across the grid. You can change the width, height and cell size at any time, and click "Change values". The grid is constructed as a one-dimensional array displayed as lines of length 'width', so if you change the height while the process is going, further blank lines will be added at the bottom. But if you change the width, cells which were adjacent to each other will no longer be.