The engineers watched as a giant machine crushed a 20 - foot - tall hollow metal can with nearly a million pounds of force over the course of about five hours. (discovermagazine.com)
And, because hollow metal allows air to get through easily, replace outdated aluminum frames with wood alternatives whenever possible. (rismedia.com)
These boats are built on pontoons (usually hollow metal cylinders) with flat surfaces to walk or sit on. (progressive.com)