Boyle’s law – The pressure of a given mass of a gas is inversely proportional to its volume at a constant temperature.
If the air pressure surrounding a balloon decreases, the balloon will expand.
In the climb – Expanding trapped gasses in the body can cause abdominal pain or in rare cases teeth pain. Burping or flatulence can be used to relieve these conditions.
In a decent – Compressing trapped gasses in the body can cause middle ear pain or sinus pain. The Valsama maneuver can be used to relieve these conditions.