While cooking food in my pressure cooker today, I pulled the whistle using my spatula to let the steam off before I opened it to check on how well cooked the food was. Before, I opened the pressure cooker, I let the whole steam drain out. I opened the cooker, saw the food wasn't cooked enough and closed the lid again. Note that I turned off the stove while doing this.
For some reason, I tried pulling the valve again after closing the lid, and interstingly, gas came out of it. How is this possible?