The symbol for taking the factorial of a number looks suspiciously like an exclamation mark . . .

!

If you’re a factorial, answer this, sonny:
You don’t seem to me to be terribly funny,
So how come your symbol’s a fake exclamation?
You’d better come up with a good explanation . . .

“The answer, my friend, is well known to the wise.
The reason the symbol looks like a surprise
Is because we factorials are not often used,
And so, when we are, we are highly amused!”