Preach floods to Noah

Advocate to someone an opinion or stance that s/he already holds -- preach to the converted -- push on an open door . . . with a flavour of "a bit of a waste of time and effort"

Turns out we agree on this, so I won't try to preach floods to Noah.

wash a tire

Urinate at roadside during trip by automobile

I need to pull over soon -- I have to wash a tire.

Drain their potatoes

Drain your potatoes, drain our potatoes, drain his potatoes, drain my potatoes -- urinate

We need to stop soon so the boys can get out and drain their potatoes.