The Bedtime Scientist

Episode guide

The Bridge Built to Move

Every bridge moves. All of them, all the time, and that is not a fault in the building. This episode stands on a stone bridge in Greece that has held for three thousand years, and on a steel one in San Francisco that rises and falls with the temperature, and asks what strength really means.

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The Bridge Built to Move

0:00 9:29
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The science, gently

  • The Arkadiko bridge in Greece was built more than three thousand years ago from huge limestone blocks with no mortar holding them together. The stones stay put because of their shape, their weight, and the way the forces run through them. People still walk across it.
  • The Golden Gate Bridge's steel cables stretch when the sun warms them and pull back in as they cool, so the roadway slowly rises and falls. The cables are so heavy that the bridge lags about two hours behind the temperature.
  • Across its whole designed range that roadway can travel about sixteen feet between its lowest point and its highest. It is not failing when it does that. It was drawn that way.

Wonder together

  • What do you think would happen to a bridge that was not allowed to move at all?
  • A bridge holds up cars, trucks, wind and its own enormous weight. Where do you think all that pushing finally goes?
  • Would you rather cross the bridge made of stacked stone or the one made of stretched steel?

Tonight's calm-down

A bridge spends its whole life holding something up, and it does it by giving a little. It sways when the wind comes, stretches in the heat, settles again in the cool of the night, and none of that is it failing. It is how it holds. You are allowed to hold things the same way tonight, loosely, with a bit of give in you, and still be strong enough.
Go further together

The wondering doesn't have to stop at bedtime.

The episode is a whole story on its own. But if the wondering keeps going after the audio ends, the Family Guide opens this one all the way up: the same calm voice, turned into something you can do together at the table the next morning.

  • Questions written for your child's age, not just their reading level
  • One hands-on experiment, simple enough for tonight
  • The science words behind it, in plain language
  • A printable to color, keep, or fill in

Two versions inside: one for ages 4–6, one for ages 7–10.

Open the Bridges Family Guide See what's included

Free to read while the library grows.

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Tomorrow night

If you are still wondering, back up to the surface, and the heaviest thing that rides on it.

Floating Ships

Why the heaviest things rise

Ideas inside this one: force and machines.

I like your mind. Just the way it wonders.