You place the two cats on a wooden board, side by side. You then raise on end of the board. As you raise it, one of them slides off. Why?
Clearly, one of them had a greater µ ; )
µ is the Greek letter mu. It's the coefficient of static friction. It's also pronounced "mew" like the sound a cat makes.
Static friction is the term for the force a surface applies to an object by friction which keeps it still. That force is equal to µ * N, where N is the normal force (the equal and opposite force the surface applies to the weight of the object). Specifically, the force is in the range of 0-µ*N. If the force necessary to keep an object from moving exceeds µ*N, then it slides off.
Having a greater µ in this case is a pun on the sound a cat makes (mew) as well as a higher coefficient of static friction.
EDIT: added an HR to separate the explanation from the joke.