In Chapter 18 of "Paper Towns" by John Green, Ben suggests that Margo might be keeping herself in "Judaism" or "inside a coffin," as part of the ongoing mystery surrounding Margo's disappearance.
In "Paper Towns" Chapter 18, Ben's suggestions about where Margo might be keeping herself are meant to be humorous and exaggerated. The characters are engaged in a search for Margo, following the clues she left behind. Ben's humorous remarks, such as suggesting she's in "Judaism" or "inside a coffin," highlight the absurdity of the situation and the difficulty the characters face in understanding Margo's intentions.
The novel explores themes of identity, perception, and the complexity of human relationships, and Ben's comments contribute to the overall tone of mystery and discovery in the story.