A popular math joke is:
 
Q: Two cats are sitting on a sloping roof. Which one falls off first?
A: The one with the smallest mu.
 
The Greek letter mu stands for the coefficient of friction. The cat with the smallest mu—the least friction—will fall off the roof first. Also, cats say “mu” or “meow.”
 
The math joke has been cited in print since at least 1993.
 
   
Wikipedia: Coefficient of friction
A coefficient of friction is a value that shows the relationship between the force of friction between two objects and the normal force between the objects. It is a value that is used in physics sometimes to find an objects normal force or frictional force, when other methods aren’t available.
 
The coefficient of friction is shown by f = \mu F_{n}\,. In that equation, f is the frictional force, \mu is the coefficient of friction, and F_{n}\, is the normal force.
 
Google Groups: eunet.jokes
Scientific…
S.J.Soloman
8/30/93
I apologise on behalf of my flatmate for the following…
 
Two cats are sitting on a sloping roof.  Which one falls off first?
 
The one with the least mu.
   
Google Groups: rec.humor
Funny Name: Football Player
D.J. Wischik
11/10/93
(...)
Q. Two cats are sitting on a tin roof, and one falls off. Which one stays on?
 
A. The one with the larges mew.
(greek letter mu = coefficient of friction)
 
Google Groups: eunet.jokes
Just a quickie, with a longie attached
Paul G Roberts
6/5/94
BEWARE: Specialist Joke.
I’ve been reading this splendid forum for ooh ages now (two days) and I would like to offer up the following, which has probably been seen before.
 
Q: Two cats are sitting on a shed roof.  Which one falls off first?
A: The one with the smaller mu.
     
Everything2
Two Cats on a Roof
(idea) by The Big D Sat Nov 13 1999 at 10:24:38
Which one falls off first?
The one with the smaller mew.
he he. ha. ahh. mm.
Well, in England, at anyrate, the Greek Letter mu is used to denote the coefficient of friction.
sorry about that - have a look at some other geek jokes.
   
The Geek Culture Forums
Terrible Maths Joke
Posted by spungo (Member # 1089) on April 22, 2004, 09:16:
Another crap maths joke:
 
Two cats sitting on an inclined roof - which one falls off first?
 
ans: the one with the smallest mu
 
Twitter
Marcus du Sautoy
‏@MarcusduSautoy
Two cats on a roof. Which one slips off first? The one with the smallest mu. (mu is the symbol for coefficient of friction)
7:29 PM - 14 May 2009
 
YouTube
Math Jokes Explained - Numberphile
Numberphile
Published on May 20, 2013
Some of your favourite maths jokes are dissected in forensic fashion.
2:58
“Two cats are standing on a roof. Which one falls off first? The one with the smaller mu.”
 
Nerdy Jokes
May 7, 2015
A Furry Friction Funny
By LC
Q. Two cats are sitting on a roof.  Which one slides off first?
A. The one with the smaller mu!