Use Quicklook in Stacks

Quicklook is an amazing feature of Mac OS. Highlight an item, and press the space bar to get a peek at the file. Whether it's a document, or an image, you can see the contents without having to open the program associated with it. With Mac OS Lion, you can use Quicklook right from a Stack on your Dock. If you're using Fan or Grid view, you can use Quicklook in a very similar  way you would for any other file. Just hover your mouse over it, and press space. It's that easy. Just another one of those 250+ features that have been added to Lion. 

via CultOfMac