The Four-Walled Fortress

I found four square cork coasters stacked in a sloppy pile on the coffee table. Completely wasted structural potential! I shoved the stack flat across the dark wood and set to work immediately. Up goes wall number one! Wall number two props right into the corner. Three and four snap together like absolute magic. Zero nails, four clean right angles, and zero wobbling! I leaned my arms over the rim and peered straight into my new enclosed perimeter. A magnificent hollow fortress, constructed in record time!