How to add spaces between dock icons?

At school they have it set up where there is a space between sets of icons. For example: Form-Z, SketchUp, Maya, 3DSM are together (normal spacing), then there is a larger space, then all the icons for Adobe CS3.

How do you get that space in there? I'v right clicked it and it appears to be a blank "something" or a hidden "something."

Whatever that something is....how do you do it?