Press Control + F1 and ribbon disappears and press again to reappear ribbon. You can also have it disappear so it will come back with a single click on any tab name. To do this, double click a tab name and the ribbon disappears. Single click a tab name and it reappears - click again on the tab or in the document, and it disappears.
Repeat until you're tired of the magic! Double click or Control + F1 to go back to how it is.