Rosehip Mole asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
speed of sound
Sound isn't instant — it's a wave of jiggling molecules, and that jiggle takes real time to travel.
In air, at room temperature, sound moves about 343 meters per second — that's over 1,200 kilometers an hour.
Lightning flashes instantly since light is nearly a million times faster — but the thunder's boom lags behind, arriving later.
Count the seconds between flash and boom, divide by three, and you get the distance to the storm in kilometers.
Sound speeds up in denser, stiffer stuff — molecules packed close pass the jiggle along faster than in air.
Sound is a chain reaction of bumping molecules, and how packed those molecules are decides how fast the message travels.
What breaks the sound barrier?
When something flies faster than sound itself — faster than 343 meters per second in air — it breaks the sound barrier.
Below that speed, the jet's sound waves spread out ahead of it, like ripples racing away from a boat.
But once the jet catches up to its own sound waves, they pile up into one crushing wall of pressure — the shock wave.
That cone sweeps across the ground and we hear it as one sharp crack — the sonic boom — not a rumble but a slap.
It's not a one-time event either — anyone under the plane's path hears their own boom as it passes overhead.
Breaking the sound barrier means outrunning your own noise, so all that piled-up sound slaps the ground as one loud boom.