If you click on a file, folder or image in Mac OS X and then press the space bar to see a QuickLook screen. Here, you can click on the diagonal arrows pointing outwards to move into full screen – great for showing documents or images during presentations. While in full screen mode, click on the diagonal arrows again to go back to a windowed appearance. If it is an image then you can will also have an icon to save the image into iPhoto and can option-click on the image to zoom in.
