Caution: dirty words ahead

Here's the kind of stuff that keeps me up at night.

Why do they call it 'taking a dump'?
I don't know about you guys, but for me, the prospect of taking this somewhere (and where would you take it?) doesn't have much appeal.

There's a similar expression 'taking a piss', again suggesting something that only Tour de France cyclists need to do. Surprisingly, 'taking the piss' means something completely different.

Does it have to do with the subject matter? Because comedian Ben Elton once noted:.
"Blow job.. BLOW job. If there's one thing anyone knows, it's... you are NOT supposed to BLOW."