We talk about boiling retardation if under certain conditions liquids are heated up to temperatures above the boiling point without boiling. Water can be heated to 110 °C without forming any vapour bubbles by evaporation. The metastable state can, however, lead to a risky situation since already small vibrations might form a bigger gas bubble which will deflagrate from the vessel. This effect will mainly occur in high, narrow vessels, e.g. test tubes, since the liquid has a higher temperature at the bottom than on the top. Boiling retardation is favoured by a high purity of the liquid substance as well as by local overheating caused by poor mixing and plane and smooth walls of the vessels. Possible remedies are stirring equipments, boiling stones or boiling capillaries.