If you haven't thrown out the About Stacks.pdf in your Documents folder, open it and you'll see there's a paragraph that says: But when you do that, the Stack opens instead of showing a menu. To access the menu, you have to right click instead. I filed a bug report to Apple using the Feedback form when it was version 10.6.1. Yet today we are at 10.6.7 and this still hasn't been fixed. Some might tell me to "just right click", but this is clearly an inconsistency issue and should be fixed.