"Does the bear s**t in the woods"?

The earlier thread about onions led me to think about expressions in English that use a unquestioned known truth to express the truth in another statement. There are a number of these, but one used in the midwest part of the US is the title of this thread. It is used when an individual says something is true -- and compares it to a 'natural truth'. One which is less than PC today is 'is the Pope Polish?'.
Are there other examples from the English speaking world of these types of expressions?

That the current pope is Polish is a contingent truth rather than a "natural truth".
That triangles have three sides is not a contingent truth and the rhetorical question
'Does a triangle have three sides?' does the job you're referring to.