12 Math Knock-Knock Jokes

In a very old Second City skit, a man on hold complained (to no one in particular) about the hold music. After his complaint, a voice on the other end of the line said, “I’m sorry. Don’t you like my singing?”

“Who are you?” he asked, surprised.

“I’m your hold operator. If you don’t like music, I’d be happy to entertain you in some other way. Would you like to hear a joke?” she asked.

“Um… sure, why not?”

“It’s a knock-knock joke,” she said. “Are you familiar with the format?”

Now, that’s just funny!

My favorite joke to tell in the classroom is a knock‑knock joke, so I hope that you are familiar with the format.

Knock, knock.
Who’s there?
Interrupting cow.
Interrup — ?
Moo!

My sons are now of an age where they can understand jokes, and those of the knock‑knock variety are told daily in our house. (The knock-knock jokes at GRiN are a source of endless amusement.) Sadly, I didn’t know any knock‑knock jokes that are mathy… so I made some up. Here they are, 12 totally original (sort of) but not terribly funny math knock-knock jokes. Aren’t you glad you stopped by today?

Knock, knock.
Who’s there?
Lemma.
Lemma who?
Lemma in, it’s raining!

Knock, knock.
Who’s there?
Mode.
Mode who?
Mode the lawn. What should I do next?

Knock, knock.
Who’s there?
Slope.
Slope who?
Slope ups should stay on the porch.

Knock, knock.
Who’s there?
Convex.
Convex who?
Convex go to prison!

Knock, knock.
Who’s there?
Prism.
Prism who?
Prism is where convex go!
(Weren’t you paying attention to the previous joke?)

Knock, knock.
Who’s there?
Origin.
Origin who?
Vodka martini origin fizz?

Knock, knock.
Who’s there?
Zeroes.
Zeroes who?
Zeroes as fast as she can, but the boat doesn’t move.

Knock, knock.
Who’s there?
Unit.
Unit who?
Unit socks; I knit sweaters.

Knock, knock.
Who’s there?
Outlier.
Outlier who?
Outlier! We only let honest people in this house!

Knock, knock.
Who’s there?
Möbius.
Möbius who?
Möbius a big whale!

Knock, knock.
Who’s there?
Tangents.
Tangents who?
Tangents spend a lot of time at the beach.

Knock, knock.
Who’s there?
Axis.
Axis who?
Axis for chopping, saw is for cutting.

What’s the Difference?

Today is 8/20/12, which makes it a difference day, because the difference between the date and the month is equal to the year: 20 – 8 = 12. In general, a day in the form mm/dd/yy is a difference day if dd – mm = yy.

Dates of this type are relatively rare. There will be exactly 12 per year through 2019, but from 2031 through 2099, there won’t be any. So here’s a question:

How many difference days will there be during the 21st century?

The answer is below.

Speaking of differences, here are some math jokes about differences.

What’s the difference between a narcoleptic and a math professor?
The narcoleptic is a slumber nut.

What’s the difference between a math Ph.D. and a large pizza?
A large pizza can feed a family of four.

What’s the difference between a lemma and a proposition?
You’ll never receive a lemma at a bar.

What’s the difference between a mathematician and a chocolate muffin?
One is a mathematician, and the other is a chocolate muffin.

Though I believe mathematicians are useful, I would much prefer a machine for turning theorems into coffee.

(Admittedly, that last one is in the wrong format. But it seems weird to ask, “What’s the difference between a mathematician and a machine for turning theorems into coffee?” The answer would be, “Nothing.”)

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There will be 281 difference days during the 21st century. There are 12 per year for 2000–19, but then the number per year starts to decrease. You might expect there to be 11 difference days in 2020, 10 difference days in 2021, and so on, with the number decreasing by 1 each year. But February, April, June, September and November cause problems because they have fewer than 31 days. So the total number of difference days during the 21st century is:

20(12) + 10 + 10 + 8 + 8 + 7 + 5 + 5 + 3 + 3 + 1 + 1 = 281