A flask is a type of container made of glass. They are a common piece of apparatus used in laboratories. Their size are chosen by the volume they can hold, and is measured in metric units, like litres (L) or mililitres (mL). Their shape depends on what they are used for, but they usually consist of a wider body, which contains the materials, and a more narrow neck with an opening at the top. The opening is the way to insert materials into the body. Some flasks have more than one neck and opening at the top.