Changed from Susan's original to close submenu if (1) the header is clicked again, so it acts as a toggle, or (2) anywhere else is clicked. If a submenu is open and another header is clicked, the first is closed and the new one is opened.

This will involve teaching them about

• event propagation
• the "event" object argument to event handlers
• testing whether a submenu is open
• attaching an event to the document