Aluminum is a lightweight, corrosion-resistant metal with forms ranging from angles and channels for structural support to foil sheets and rolls for packaging and wrapping. Aluminum bars, plates, and sheets are solid pieces that can be machined into metal parts and other items. Aluminum expanded and perforated sheets have openings that allow light, air, and water to pass through. Aluminum grating and stair treads provide an alternative to steel grating for walkways, mezzanine platforms, and other flooring applications. Aluminum tube stock (available in rectangular, round, and square shapes) has a hollow center and is often used for framework, fencing, and support posts.