-Is this thing on?

[TAPPING]

[COUGHING]

-So a man walks into a bar.

He asks for 10 times more drinks than everyone else.

The barman says, now, that is an order of magnitude.

MATT PARKER: Actually quite a good joke, because order of

magnitude is how big a number is.

The speed of light is 3 times 10 to the 8 meters per second,

and that's the order of magnitude.

It's 3 with eight 0's after it.

And something like a kilometer is 10 to the 3 meters.

So light in one second goes five orders of magnitude

further than a kilometer.

And so when he says, I want 10 times as many drinks, he's

actually ordering one order of magnitude more than anyone

else, which is, of course, an order of magnitude.

-So an infinite number of

mathematicians walk into a bar.

The first one, he orders a pint.

The second one, half a pint, then a

quarter, then an eighth.

Eventually, the barman hands over two pints and says, you

mathematicians.

You just don't know your limits.

MATT PARKER: OK, so if you start with 1, and then you add

1/2, and then you add 1/4, and then you add 1/8, and each

time you're getting smaller and smaller, what we're

actually doing is we're summing all

the 1/2 to the n's--

some n.

And we start with n equals 0, and we go all the way up to a

correctly drawn infinity sign.

And if you carry this on infinitely, its limit equals

2, which is why the barman gave them two pints.

And he said, you don't know your limits.

Because that's the limit.

-So why is 6 afraid of 7?

Because 7 ate 9!

MATT PARKER: Not strictly a math joke, but OK.

The thing here is that numbers appear in order.

They go 1, 2, 3, 4, 5, 6, 7, 8, 9.

Why is 6 scared of 7?

Well, when you say 7, 8, 9, the word "eight" in English

sounds a bit like the word "ate." So it

sounds like 7 ate 9.

So eight is, in this case, a word.

And 7 then eats--

I don't know.

I mean, I personally would say that 7 is a 6 offender, but

I'm not the one doing the jokes here, am I?

-How do you make seven even?

Remove the S!

MATT PARKER: I was promised number jokes, but I guess

another English one is fine.

So the number 7--

if you write it out in this language called English, which

you may have heard of, you spell it "seven." And then if

you remove the S, you're left with the English word "even."

No actual math's involved.

-Two cats are standing on a roof.

Which one falls off first?

The one with the smaller mu.

MATT PARKER: OK, bear with me here.

I'm going to represent the roof like this.

The front door and chimney are optional.

And then I'm going to approximate

the cat as a rectangle.

Gravity is forcing the cat directly down.

But of course, it can't go straight down because it's on

a sloped roof.

And so you're going to split this into a normal vector,

which comes off this way, and then the vector

going down the roof.

What stops it from falling off the roof is friction.

And friction will be a force coming back this way.

And friction is proportional to how hard the box is being

pressed down into the roof.

And so we have the coefficient of friction, which is the

Greek letter mu, times whatever that normal force is.

And so the bigger the coefficient of friction, the

less likely it is to fall off.

And if there were two boxes, the one with the smaller

coefficient of friction would fall off first.

And that is the Greek letter mu, which, if we're talking

about cats, the cats make the sound meow.

-What did the number 0 say to the number 8?

Nice belt.

MATT PARKER: OK, I guess so.

So 0 looks a bit like this.

And if you imagine putting a belt around 0 and then

tightening it, it would squeeze the middle

of 0 in like that.

And if you tighten it enough, it would pinch off and

would form an 8.

And so an 8 is a bit like a 0 wearing a belt.

Why it is a nice belt, I'm not sure.

[MOSQUITO BUZZING]

-What do you get if you cross a mosquito

with a mountain climber?

Nothing.

You can't cross a vector and a scalar.

MATT PARKER: Ok, there's a lot going on here.

We'll start with what we mean by cross.

And it's a way of saying multiply.

When you multiply numbers together, it's easy.

There's one thing you do.

You multiply one by the other.

When you multiply vectors, it gets a bit more complicated.

So let's say I've got one vector u, which has ux

component, uy component, and uz component,

because it's in 3D.

And then I've got another vector I'm going to call v. So

I've got vx component, vy component, vz component.

And then I want to multiply these two vectors.

And there's more than one way to do that.

You can dot multiply, which gives you a scalar, which is

just a normal number, out as an answer.

Or you can cross multiply--

using our traditional cross multiply symbol--

which gives you out another vector.

Now, cross multiplying is a little bit complicated.

OK, let's do the first one.

So it's uy normal multiply vz minus uz normal multiply vy.

Then the next component-- and you keep going.

And you get three new components out this way.

The trouble is, you can only cross multiply vectors.

You can't do the same thing for normal numbers, or

scalars, as we call them.

And someone who climbs the mountain is scaling it.

So I guess you could call him a scaler.

And a mosquito--

we're going right outside maths here-- a mosquito can

transmit diseases, which in biology, you would call a

vector as a way of transmitting diseases.

And so you can't do a cross multiplication between a

vector and a scaler.

And so that's the joke.

-Did you hear the one about the constipated mathematician?

He worked it out with a pencil.

MATT PARKER: No.

We're aware that there are other maths

jokes we've not covered.

We're also aware that some of you have discovered the

comments section underneath YouTube videos.

So if you have your own maths joke you'd like to contribute,

we'll not stop you.

It's definitely going to happen.

And we will endeavor, if we get enough interesting ones,

to explain the maths behind more maths jokes.