I'm not very good at this but I'll see if I can explain my thought process:

(is tiger fed) requires (eats tiger ?food)

which means there must exist ?what such that (is ?what eatable) and (is ?what ?food)

But in the fact table, nothing is actually eatable! We could have our tiger eat plants by asserting (eats tiger plants) and (is grass eatable), but that's two assertions. Tigers eat animals, and tigers ARE animals, so since we need to make something eatable we might as well do it on the only animals we have: (is tiger eatable)

I got level 2 as well but only after checking for hints in the source. I have been thoroughly nerd-sniped.

Okay, so it's not a typical tutorial. You have to golf and hack it. I'll give it a go again. Thank you for your explanation. :)

yesh, treat it more like a "reverse engineer a broken system" challenge than a "real world problem solving" challenge.