1. Create a folder and put something in it (unless you already have a folder with contents).
2. Collapse that folder, but leave it visible.
3. Drag a placemark (or any type of GE object that can be placed into a folder) to the now collapsed folder.

GE will refuse to put the item into the collapsed folder.

BTW: GE should expand folders when you hover while dragging. That allows you to put the item being dragged inside a subfolder. Windows Explorer does this.

Finally, it would really help when I rearrange items if the places pane would scroll as needed while dragging. GE is the only program I have seen that does not do that.