Dear student
1. Gases occupy the entire space of the vessel in which they are kept because there is a lot of space between gas molecules which are always moving and reach each and every corner of the vessel. Therefore, a gas cylinder cannot be half filled.
2. In a pressure cooker, the vapor pressure over the water surface is more than the atmospheric pressure. This means boiling point of the water will be higher in the pressure cooker than in the open. This will let the cereals and food to be cooked in higher temperature than at 1000C,. Thus, the cooking process gets faster.