[stoneman24]

To extend D Rumsfeld

there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know.

And now things that we know and wish that we didn’t (ie spaceship recycling)

[whycome]

Pee is largely recycled right now on the ISS. It's mostly water.

And nitrogen is good for growing plants - the cellulose of their structure is the ideal insoluble fibre for us. Though, cows turn it into usable sugars.