You’re sitting on a plane destined for a far off place, sandwiched between two people, and you're doing your best to sit back and get comfortable. But as the plane takes off and makes its rapid ascent toward cruising altitude, a baby begins to cry, the people around you wince, and finally, it hits you—a buildup of pressure, tightening your ears and sinuses, that compresses your head like a vice that won’t let go. The plane continues upward until it stabilizes thousands of feet above the ground and then—POP!—your head feels fine.

Whether you’re in an airplane soaring through the sky, on an elevator heading to the top floors of New York’s tallest skyscraper, or making a deep dive underwater, your ears will most likely pop. The explanation for why this occurs is simple: It’s pressure. But what, exactly, is happening inside your ears?

Under Pressure

When a plane ascends, the air pressure in the cabin lowers at a rapid rate. This sudden change causes an irregularity with the pressure in the inner ear. At such high altitudes, the pressure pushes outward on the eardrum—the thin membrane between the external and middle ear that transmits sound—and causes the tension you feel in your head. (The pressure also reduces your ability to hear.)

One way to release this pressure is through the Eustachian tube, a 1.4-inch long cavity in the middle ear that connects the ears to the nose and throat. Yawning, swallowing, or even chewing gum opens the muscles of the Eustachian tube, causing air to fill the space and equalize that sometimes debilitating pressure caused by rapidly changing altitude. During that equalization, the air forcing into the tube makes that pesky popping or crackling sound, alleviating some of the discomfort the fluctuation caused.

Fancy Maneuver

When yawning or swallowing doesn’t do the trick, people use what is known as the “Valsalva Maneuver.” Named after Antonio Maria Valsalva, a 17th century Italian physician whose scientific specialty was the ear, the maneuver consists of closing the mouth, pinching the nose, and exhaling as if to blow up a balloon. It isn’t recommended, however, as it may cause barotrauma—damage to bodily tissue caused by a pressure difference inside and outside the body—or further auditory damage from the violent pressure equalization pushing outward.

After you’ve heard that pop, the pressure should be equalized, and the pain gone. You can watch some in-flight entertainment, or chow down on the packet of peanuts the flight attendants give you—at least until the descent, when, thanks to the rapidly increasing pressure in the cabin, you might have to go through the discomfort, and popping, all over again.