When Ma tried to explain what world looked like and said
the world is so big and the room is just one stinky part of it.
Jack cried, yelling
the room is not stinky only when you do a fart.
Why did jack cry? Can someone explain?
Jack cried because he doesn't want to accept the truth.
When he was little, Joy fed lies to him that they get food from TV by magic. And he doesn't know that there's a real world outside, real living animals, oceans etc. He believes that the people from the tv, animals and the world outside is not real.
Now that he's five years old, Joy tries to explain him that she was tricked by Old Nick and he locked her up in the room for 7 years and there's real world outside. Suddenly, it's too much for him to absorb, whatever he believed in the last couple of years are lies, he still wants to stick to those lies because he's comfortable with the lies because he's a kid.
Generally, kids are stubborn to let go of their theory that they believed in. After joy explained everything, he's angry at her that the things he believed are lies and it's too much for him to accept the truth.