Here's the long answer


is also undefined, since you can't divide by 0.
The reason is that when we write

, we want

to be the
unique solution of the equation

.
In the case of

(i.e. dividing by zero) the equation is

, and if

is nonzero, it's impossible to find an

which will make that work.
So what if

? Well, then you have to solve

- meaning

could be anything you like. But then that's not a
unique solution, so it goes against what I said above.
So why do we want it to be unique?
Well, what if we just pick one of the possible solutions, and say

?
Then multiplying by 2,

So trying to just use

leads us to say

, and the universe blows up

The same thing would happen if we tried to make

any other fixed number. So we just agree not to