Because all kinds of oil in contact with air, will produce oil vapor. When the heat of oil and air are mixed in a certain proportion, an explosive mixture will occur in the event of an open fire. The explosion limit can be divided into an upper limit and a lower limit. When the oil vapor in the mixed gas is below the lower limit of explosion, it will not explode and cannot burn: when the upper limit of explosion is exceeded, the mixed gas will not explode and can burn slowly. After the fire, if some conditions change, an explosion is still possible.