I think I've pissed myself

I have a 12" penis, but I don't use it as a rule

a 12-inch metal rule
A 30.5cm rule.

Those who can, do; those who can't, teach. Oft said, but it doesn't really apply to driving instructors. Or at least I hope it doesn't!

For every rule, there's an exception. Oft said, but if it's true then there must be a rule for which there is no exception.

But what is that rule? (thinking)

Perhaps there's an exception for every rule is the rule without exception? That way, all other rules can have one. (pipe)