In Mac OS X many dialog boxes slide out of a window's title bar and are attached to that window rather than being on their own. These dialog boxes are know as "sheets" and can be anything from Open/Save dialogs to the Customise Toolbar settings. They are attached to a certain window to show that they are associated with that particular file or application. If you can't remember seeing this before, open up a TextEdit file and choose Save As.

The sliding animation may be pretty, but it can waste a fair amount of time if you are opening and closing sheets all day. You can speed this up and make your Mac feel much snappier by using a simple Terminal command to reduce the amount of time taken to display the animation.

Launch the Terminal (located in Application/Utilities) and type the following command:

defaults write NSGlobalDomain NSWindowResizeTime 0.001

The number at the end is the length of the animation in seconds. With it set to 0.001, sheets appear pretty much instantly. The default is 0.2, so type the command with 0.2 at the end to return your sheet animation to normal.

For fun you can try higher values. This allows you to see just how detailed the animation is. Type the command with 2 at the end and load up TextEdit. Resize the window to as small as possible before selecting Save As from the File menu. You'll see the sheet jump up out of the window then gently fall into place.