Question

why should we cover fry Pan while cooking or use pressure cooker will it save cooking gas

Answer 1

Water in the food your trying to cook has a certain boiling point. This boiling point is elevated with increase in pressure (since under high pressure, vapors tend to condense back). To cook the food properly, you'd want the water to stay as liquid as much as possible, even under high temperatures, due to the fact that liquid water has a high specific heat capacity, storing lots of heat energy for a given temperature and can transmit heat to food better. Under pressure, boiling point of water is increased, so that it still remains liquid. This helps cook the food faster than without pressure.