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A sign at a Middletown, Conn., deli caught Anna Haensch's eye. It read, "Please refrain from discussing mathematics while waiting in line."

Haensch, who teaches mathematics at Pittsburgh's Duquesne University, knew there had to be a good story behind the sign. So she tweeted a photo of it, and her picture went viral. That's when she did some sleuthing of her own.

Haensch spoke with As It Happens host Carol Off about what she found out.

CO: What went through your mind when you saw a sign in this deli that said, "Please refrain from discussing mathematics while waiting in line"?

AH: Well, first I thought what on Earth could have happened here to require the making of a sign to prohibit people from talking about mathematics? Immediately, you can imagine, there must have been some kind of a brawl over fibonacci numbers or calculus or something.

CO: And you're a mathematician so this would set off separate alarms than it might for other people.

AH: Oh yeah, for sure. And, actually, the deli's in Middletown, Conn. And I'm here on a research visit right now and I was actually just taking a break from working with a collaborator here and I walked down to the deli to get a sandwich. So I was thinking about math when I saw the sign, luckily I didn't think it out loud though.

CO: And did you ask anyone at the deli why the sign was there?

AH: I did, yes. The deli is just down the road from the math department here at Wesleyan, it's called the Neon Deli. It's kind of a popular spot for the students and faculty here. So I knew that a lot of the math faculty went there for lunch and in fact there are several sandwiches named after the math faculty. So I kind of assumed that maybe they had something to do with it and indeed that was the case.

CO: What are some of the sandwich names?

AH: Dave's Tasty Turkey. It's just a sandwich named after Dave Pollack who's a professor here. And it's just a turkey sandwich with roasted peppers. Just like a non-mathematician would eat.

CO: So there's nothing particularly mathematical about these sandwiches?

AH: No, only in so much that mathematicians like to eat them.

CO: And mathematicians line up to buy them I presume?

AH: Certainly they do. And often it's the case that people are walking down from the math department on their lunch break and typically you're ... engrossed in whatever you've been working on and you … don't want to stop. We will often be down there having talks about mathematics. And I think, well I know actually, that sign was inspired by Dave, of Dave's Tasty Turkey sandwich, and my own PhD advisor.

They were down there talking about math one afternoon and the owner of the deli was trying to take the order from one of them and he was a little bit too preoccupied with mathematics to fully engage in the sandwich ordering process.

CO: So Dave was talking mathematics in line and they couldn't get his order?

AH: Right. And they said then "please refrain from discussing mathematics while waiting in line." Just because it was kind of a joke, they wanted to speed up the ordering process. But Cynthia, who was the owner who was there that day taking sandwich orders, she is a good friend of the mathematics department.

CO: Did you find out all this after? Because you tweeted about the sign, right?

AH: Yes, and I saw the sign and I didn't even ask, actually, I just took a picture of the sign. Honestly, I didn't think anyone would be very interested in it. I thought it was a little bit funny and I thought maybe five other people would find it funny. Then I went to sleep that night and woke up the next morning and it had been retweeted 30,000 times. I thought "Oh my gosh, what on Earth?" It's just kind of touched a nerve with people.

So at that point I went into my advisor's office and asked him what he knew about it and he told me the story.

CO: It probably also comes as a surprise to people that mathematicians have a sense of humour.

AH: I think it does, it shouldn't, but it does.

CO: Are there math jokes? Are mathematicians secretly very, very funny people?

AH: Yes, they are. They're very, very funny.

I think that they wouldn't be funny to a non-mathematician, probably.

CO: That was my question. Are these jokes that anybody would find funny or are these just inside jokes for math nerds?

AH: No, I can give you one example of a mathematical joke. People think the following joke is really funny, that someone is giving a lecture on group theory and they stand up at the board and say "let L be a group".

CO: If you told that to a bunch of mathematicians, they would be on the floor?

AH: Yes, they would be laughing because they would say "No one lets L be a group, groups are G".

CO: I'll leave you with this one that I've just been sent. Why do they never serve beer at a math party?

AH: Oh, because you should never drink and derive. I know that one.

This interview has been edited for length and clarity. For more, listen to our conversation with Anna Haensch.