turn autosave on

I have the latest versions of iWorks apps and am running OS 10.8.2.

For some reason, when I make a change in a document in Numbers or Pages, and then close the document, it does not automatically save the document like I think it used to.

Instead, I get a dialog box that reads "Do you want to save the changes made to the document "document name"? You can revert to undeo the changes since you last opened the document...

Followed by three buttons: "Revert Changes" "Cancel" "Save"

I can click on Save and it will save the document but I thought this was no longer necessary under the latest versions of iWorks and Mountain Lion?

Any suggestions would be appreciated...

thanks