(upbeat music)
- This is an assassin's teapot.
It can be used to pour yourself a drink
and to pour a drink for your enemy.
You can then down your drink
to prove that the drink isn't poisoned.
There's no poison there.
Don't worry about the poison, there is no poison.
But then when your enemy takes a drink,
they find out that it is poisoned.
And by the way, this doesn't require you to spend
the last few years building up an immunity to iocane powder.
It works straight off the bat, no preparation required.
That's because the assassin's teapot
can be used to pour three different drinks,
which I've shown here using three different colored liquids.
And by the way, if you plan to poison your enemy,
I recommend using liquids of the same color.
Do I need to tell the viewers that they shouldn't
be trying to poison people, legally?
Just 'cause it would break the flow of the video.
Film myself having a fake phone call with a lawyer
in which I explain the problem,
and so long as I say out loud, "Don't try to poison people,"
then that should have me covered?
Okay. Click.
The way you choose the drink is by sneakily
covering up one of these holes.
If you cover this hole, you get a blue drink.
If you cover this hole, you get a yellow drink.
If you cover no holes, you get a green drink.
So three drinks in total.
Though, actually no, four drinks.
If you cover both holes, you get the null drink.
But how does it work?
Well, you know me, if I want to describe
how a hydrodynamic mechanism works,
I like to make a transparent 2D version of it.
I'll put a playlist of those videos
in the card and the description.
First, I needed to figure out
what was going on inside the teapot,
and a bit like with the gluggle jug,
the only really sensible way to do that
was to hit it with a hammer.
So you can see there are two chambers here, side by side,
and they both lead up to the spout.
Additionally, one chamber leads up to this hole,
the other chamber leads to this hole.
For the 2D version, I don't want to put them side by side,
'cause then it wouldn't be 2D, but I feel like
I can put them top to bottom and it should still work.
And here it is.
So if I want a blue drink, I cover this hole.
If I want a yellow drink, I cover this hole.
And if I want a green drink, I cover neither of the holes.
And for completeness, here's the null drink.
Similarly, I could have a black coffee or I could have milk,
or I could have a milky coffee.
So that's how it works, but why does it work?
Why is it that putting your finger over different holes
causes different liquids to flow out?
What it comes down to is surface tension.
it's similar to this demonstration you might've seen before.
Look, I can pour water from this bottle,
but if I turn the bottle upside down, no water pours out.
The trick is that there's a gauze
covering the opening of the bottle,
so when I pour it like this, water can flow out
and air can flow in, but when I turn the bottle upside down,
for water to flow out, air would need to flow in
and for air to flow in,
surface tension would need to be broken.
Here's another example where instead of a gauze,
there's a small hole.
The hole is actually big enough to push a straw up into,
and it floats to the top of the bottle.
It's a really cool trick.
You know, strictly speaking, ultimately,
it's not surface tension That's keeping the liquid in place.
It's air pressure.
The assassin's teapot and these bottle tricks
wouldn't work in a vacuum.
So the reason the liquid doesn't fall out
of the assassin's teapot when you have your thumb
over the hole is because the atmosphere is pushing on it.
Atmospheric pressure is holding the liquid in place.
And that's counter-intuitive.
You'd think, well, the real explanation
is that if the liquid did fall out a little bit,
it would create a vacuum and the vacuum
would pull the liquid back in.
But there is no pulling force,
there's only pushing force from the atmosphere.
It's like when you suck on a straw.
It's not the suction that's pulling the liquid up the straw.
It's the atmosphere pushing the liquid up the straw.
And that's really counter-intuitive because you feel like,
well, look, that's me doing the work.
I am sucking on the straw.
I'm pulling the liquid up with my lungs.
At this point, I want to go off on a bit of a tangent
and talk about a type of pedantry
that I think isn't always helpful.
Like imagine you say, "I'm sucking liquid up a straw now."
I mean, it's a weird thing to say out loud,
but hypothetically, and then an annoying physicist
comes along and says, "Well, technically
you're not sucking water up a straw.
It's the atmosphere pushing it up the straw."
And then in the next breath, that physicist might go on
to talk about the flow of positively charged holes
in a semiconductor.
Well, holes don't flow and a hole can't have charge.
So why is it okay to talk about
the flow of positively charged holes?
What the reason it's okay is because it's useful.
But I would also argue that it's useful
to talk about a suction force.
so long as you keep the underlying physics in mind,
you should be okay.
Like for example, you can't just
increase a suction force indefinitely.
Eventually when you get to about a hundred kilopascals,
it stops working because you've run out
of atmospheric pressure, the thing that's really
doing the work underneath.
Similarly, I think it should be all right
to talk about the flow of the cold.
Like if I put an ice cube in my drink,
the cold flows from the ice cube into the drink.
Of course that's not really what's happening,
the warmth from the drink is flowing into the ice cube
and so it gets colder, but it's useful to talk about
the flow of cold, again, so long as you understand
the underlying physics, like you can't just
keep pumping cold into something because eventually,
you reach absolute zero.
By the way, these animations are from a video I made
about how all LEDs are secretly solar panels
and all solar panels are secretly LEDs.
The link's in the card and in the description.
Okay, tangent over.
The point is, we all know that for the water to get out,
air needs to be able get in and with your finger
off the hole, well, air can get in at the top,
but if you cover over the hole,
the only way for the air to get in is through the spout,
and that's where surface tension comes in.
Surface tension acts to minimize surface area.
You might think, well, one way for the water to get out
is if the air can sneak past it on the way,
but what would that look like?
Well, it might look a bit like this.
You've got a drip of water starting on one side.
And on the other side, you've got a bubble of air forming,
but see how that increases the surface area of the water?
And because surface tension acts to decrease
the surface area, that's not energetically favorable
and so you don't get this bubbling effect,
and instead the liquid is trapped inside the chamber.
This is also the principle behind a magic trick, by the way,
sometimes called inexhaustible bottle, or think a drink.
It's usually a bit more elaborate.
It'll have more chambers and different options for drinks,
but anyway, you can read all about it in my kids book,
"Science is Magic," available from all good bookstores.
- [child] So these are magnets.
- [Steve] Yeah?
- [Child] And they're made of metal
so they stick to magnets, and then when they drive around,
they also go around.
- [Woman] And that makes it spin?
- [Child] Yeah. It kinda goes loop the loop, like this.
- That was my daughter spontaneously explaining
the mechanics of the thing that she'd just built.
The thing she just built was from a KiwiCo project,
the sponsor of this video.
Actually my kids have had a KiwiCo subscription
since way before they approached me for a sponsorship,
so I can tell you a bit about them.
It's a subscription service where a box comes
in the post every month, and everything you need
for the project is in the box.
You never need to run off to the shops
or anything like that.
And the projects are STEM projects.
There's eight different subscription lines
for every possible age group.
Something that I haven't talked about before
that I really appreciate about KiwiCo
is the spontaneous STEM discussions that arise from it.
I'm the sort of person who, a fact will pop into my mind
and I'll say, "Hey kids, did you know
this thing about the moon?"
You know, whatever.
And it's completely out of context
and the kids are sort of like, "Well, okay, fine."
So these days I'm on the lookout for STEM discussions
that are within the context of what we're doing.
And KiwiCo is great for that because
the kids are so invested in the projects already.
They're building these things, they're excited about them.
And then as you saw with the explanation,
these STEM discussions arise spontaneously,
and it's really amazing to see.
We've got a promo for the holiday season.
Perhaps you know someone who would appreciate
a KiwiCo subscription, in which case you can get
the first month absolutely free using my special URL,
if you go to kiwico.com/stevemould.
The link is also in the description,
so check out KiwiCo today.
I hope you enjoyed this video.
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- [Child] When I touch it like that.
- [Steve] Yeah? - It spins.
- [Steve] So do you think the air is doing the same thing?
- [Child] Yeah.
- [Steve] You're saying that the air from the candle,
the candle is making the air push up on those red things?
- [Child] Yeah. - Mmm-hm.
- [Steve] Let me see.