Move the mouse pointer out over your disk. Notice that the pointer is no longer an arrow, but a little magnifying glass. Whenever you have the zoom tool active, the pointer will be a magnifying glass, not an arrow or a hand or anything else.
When the mouse pointer is a magnifying glass, your mouse's buttons
have special meanings. The left button means "zoom in right here", and
the right mouse button means "zoom out". (If your mouse is set up for
left-handed use, the buttons will be reversed, of course.) If
you have a wheel on your mouse, you can also zoom by spinning
the wheel.
We'll try all of that on the next page.