Having come from Abnegation faction, Tris has never eaten real meat before. So, when they offer her a hamburger she is amazed. She not only eats one, but helps herself to a second one. Ketchup is also a new and rare treat. Many of the other non-dauntless initiates are amazed that she's never eaten meat. Tris tells them that part of living in Abnegation is to do without personal comforts or luxuries for the good of others.
