Flasks are used to contain substances. They are usually made of glass or borosilicate glass (Pyrex). This is to ensure that the substances inside the flask do not react with the flask itself, due to the volatile nature of many chemicals (such as hydrochloric acid), They can be used to heat a substance. There are many different types of laboratory flasks. They differ in their shape and what they are used for. The most common types of flask are: