Well, this should be easy…
When I am filling out a form on a webpage I like to tab to the various fields to fill them in, since it is far quicker than mouse-clicking in each one. Yet, when I get to a box that can be checked or unchecked (“I have read the terms of service and agree…” for example), I don’t know how to check it without clicking with the mouse. So how do you check a box on a form from the keyboard?