A nice idea (Though denser bricks would work better!)
http://www.instructables.com/id/Camping-Storage-Heater-from-Bricks-Sand-and-an-Am/
Don't know why I didn't think of that...
http://www.instructables.com/id/Camping-Storage-Heater-from-Bricks-Sand-and-an-Am/
Don't know why I didn't think of that...