Objects are actually drawn onto the page one at a time. (It looks like they're all just instantly there because your computer can draw objects very quickly.) The z-order is what determines the order
that the objects will be drawn in. The ones at the back are drawn first, and the ones at the front are drawn last, right over the others.
Some objects, like video and web objects, are drawn onto a separate window surface that is permanently suspended above the page and all of the normal objects on it. We call these objects windowed objects. Windowed objects always appear in front of non-windowed objects, regardless of where they are in the z-order.
1 ) P re ss C t r l + Z t o undo t h e l a s t c h a ng e .
Pressing Ctrl+Z undoes the last change that you made to the project, which in this case was to center the three objects vertically on the page.
Note: This is the same as choosing Edit > Undo.
