The river Rhine runs through here,

and if you want to get across it, then there are four ferries.

I mean, there are bridges as well, but the ferries keep going

as a tourist attraction, as tradition,

and for some folks, an actual way to get where you want to go.

Also occasionally a fondue restaurant, because Switzerland.

But these ferries have no paddles, no oars, no engines,

nothing on board to power them.

Put them in the open ocean or a lake and they'd just drift.

And yet here, they move.

So here's how they work:

there is a wire strung across the river and the ferry is connected to it.

There's a little thing called a traveler that goes back and forth smoothly.

The ferry does have a rudder for steering, so when it's time to cross,

the captain angles the boat against the current,

and uses the force of the river itself to push the boat across.

Without that cable, it'd drift downstream, fast.

But since it can't, it just crosses the river instead.

No gasoline, no human effort, just the force of the river.

Boats like this are called reaction ferries,

and there are examples all over the world.

Sometimes the design's a little different, but the principle is always the same.

I only learned about them a few weeks ago,

and in hindsight, they seem incredibly obvious.

You have an object that needs energy

sitting on top of a massive, constant energy source

just waiting to be harnessed.

But if you'd asked me how to connect those two things before I knew the answer,

then my first thought would probably have been some overcomplicated, overengineered plan

like putting a waterwheel on the side of the river

and then using that cable to send power to an electric motor on the boat.

I mean, there is a hydroelectric dam not far away.

I'd like to think that I'd have worked it out eventually,

but I'd have probably missed the easy option to start with.

To anyone who grew up in a place with these ferries, it's a really obvious idea.

But to everyone else, it's an invention we've never thought of.

To be fair, this is only a good solution in a few places.

You need a strong, fast, steady river, like the Rhine, at a place where it isn't tidal,

and where it isn't particularly seasonal either.

But if I've missed that, then the obvious follow-up question is, well:

what else have I missed?

And is there anything out there that every single human has missed?

Maybe there is still some big, obvious-in-hindsight stuff out there that'll change the world...

just as soon as someone thinks of it.