Or: how to calm a baby on a plane
To reduce the pressure difference that the aircraft skin has to withstand at high altitudes, the pressure inside passenger aeroplanes is allowed to drop to about 60% of its average sea-level value. The only part of a human being that might be adversely affected by the relatively rapid changes in pressure that this entails is the eardrum, which can be painfully stretched or can rupture if there is subjected to too great a pressure difference between its inside and outside.
Adults are not too badly affected by this: the space inside the ear drums is connected to the throat by the eustachian tubes, and adults who do not have otorhinolaryngic conditions do not have any significant problems opening them to allow the pressure to equalise, either by yawning or by swallowing.
Babies have more of a problem. They do not know the reason for the pain in their ears and they do not know how to yawn or swallow deliberately. Nor can they understand any reason why they should try. So babies on a plane tend to cry after take-off.
Luckily, yelling really loud is another way of encouraging the opening of the eustachian tubes.
For those who find loud noises at 2000 Hz (the frequency, by the way, to which human ears are most sensitive) distressing, or who would like to spare the baby for which they are responsible unecessary pain, there is an alternative: during take-off, ascent and descent, give the baby a drink. Baby drinks, baby swallows, problem solved.
Giving babies something to drink usually involves them sucking on something: either a real live human nipple, or a rubber or silicone replacement teat on the end of a bottle. The sucking action also helps with the equalisation of pressure, and the harder the baby has to suck, the better.
Drinking from a breast requires harder sucking than drinking from a bottle. So here we have yet another reason why breast is best.