Zim Wiki - How to insert tooltip (text on mouseover)

The title says it all. If you know Zim Wiki, how does one insert a tooltip? I note that the Index on the left shows a text when you pause your mouse over a page, but I can't figure out how to do that in the context of a page.

I know I can do this with a web editor (Kompozer, LibreOffice, etc.) -- but Zim will work MUCH better for my particular application.

Thanks in advance!