If you put a tight lid on a pot, steam will accumulate under the lid, so the pressure on top of the water becomes greater than atmospheric pressure.  In order for a water molecule to escape, it must now have more energy than if it just had to escape from an uncovered pot.  (Think about trying to jump out of a swimming pool with someone outside the pool trying to push you back in).

Pressure cookers cook food more quickly because liquid molecules inside the food can be heated to higher temperatures without boiling away.  In a pressure cooker, the boiling point of water can be raised to 121 degrees Celsius (250 Fahrenheit), which translates into shorter cooking times.

At high altitudes, we encounter the opposite case: the air is thinner and the atmospheric pressure is less than at sea level.  Water molecules can escape as vapor at much lower energies, so water will boil at a lower temperature.  The higher the altitude, the lower the boiling point.

In Denver (1,700 meters above sea level), the boiling point of water can be as low as 95 degrees Celsius (203 degrees Fahrenheit).  In Mexico City (2,400 meters above sea level), the boiling point of water may be 92 degrees Celsius (198 degrees Fahrenheit).  At the top of Mount Everest (9,500 meters above sea level), water boils at 75 degrees Celsius (167 degrees Fahrenheit).  In outer space, where there is no air pressure, even cold water would boil.  Astronauts must wear pressurized space suits when they walk outside their shuttle so that their body fluids - like blood - don't boil.

Cooking at high altitudes requires either higher cooking temperatures or longer cooking times due to the lower atmospheric pressure.  In baking, increasing the temperature of the oven counteracts the lower internal temperature of the food.  In cooking on top of the stove, the lower atmospheric pressure means that liquids boil at lower temperatures, requiring longer cooking times.

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