Coolant is a mixture of water and ethylene glycol (or propylene glycol) used to regulate engine temperature. It serves two main purposes: raising the boiling point of water to prevent overheating in summer and lowering the freezing point to prevent the engine block from cracking in winter. Additionally, it contains rust inhibitors and lubricants for the water pump seal. It comes in various colors (Green, Red/Pink, Blue) indicating different chemical technologies (IAT, OAT, HOAT), which should generally not be mixed to avoid sludge formation.