I've seen two different theories, and both seem pretty plausible:
1) They both actually ate the rat poison. Sangwoo knew, and immediately threw up afterwards (why he didn't show any signs of illness), but waited a while to give Bum the saltwater to make him throw up - this way he'd suffer a little and "learn his lesson".
2) There was no actual poison. The bottle was planted there by Sangwoo to test Bum, and contained something else. Sangwoo was fine, but Bum got sick because he believed he ate poison and made himself sick by worrying about it (mind over matter).
After reading it a couple of times, I'm still not too certain which I believe more...
the 2 theory i believe is the right one cause he never asked him to clean boxes and stuf until that moment in the past he put him in the kichen (hey u start cooking), and from no where he asked him to throw stuf and medicine ,is he insane! to ask him that and the only thing have been drop the bottle . so yes when i read it i believe this is the correct answer and from that day on he start sitting and eatting with him
I never hoped for happy ending ╮( ̄▽ ̄)╭ ,will i liked it but i will never like it in real live if i read something like that i wish to burn in hell (⌒▽⌒)
i wish to him to burn in hell not me hell no ( ̄∇ ̄")

Can someone please explain to me what exactly happened with the rat poison