Glass is formed by heating sand (which is mostly made of silicon dioxide) at incredibly high temperatures until it melts and turns into a liquid. Apart from being transparent, it's inexpensive to make, easy to shape when it's molten, reasonably resistant to heat when it's set, chemically inert (so a glass jar doesn't react with the things you put inside it) and it can be recycled any number of times.

Glass is fragile and can crack, chip or break if mishandled. Changes in temperatures and humidity can cause expansion and contraction of glass resulting in cracks.

• Wipe with a soft damp cloth. DO NOT use scour pads.
• Do not use harsh chemicals or detergents.
• Use newspaper to polish the glass and prevent streaking.