Restoring Stacks?

I can't imagine what I have done, but the Stacks feature seems to have disappeared from my OS X Leopard dock, including the dashed divider. Can someone please tell me how to restore it?

Thank you!

drag the folders (usually Documents and Downloads from your user directory) that you want to act as a stack into the dock next to the trash can. it should slide to make room and you'll have stacks back all nice n' cozy.

I tried that--it snaps back into the original location with a error message stating it is needed for the OS!

Aha! Before I submitted this I found another version of the Downloads and Documents that did allow me to place them on the dock. Thanks.