We'll do the same thing for touch, so our program will work on tablets and smartphones as well.

You might think we'll just do a similar thing but with the touchmove event. But touch is always slightly more complicated than the mouse.

Touchmove will mostly work, but there's an important difference: the mouse is always somewhere, but touches come and go.

It's possible the user will touch the screen but not move their finger, so we'll need to handle touchstart events as well.