Aki usually brings the Red box (70%), Mei usually the Blue box (70%) — but they swap colors 30% of the time. A box holds tonkatsu (Chibany's favorite) or a hamburger (fine, not the favorite), and the lucky color drifts. Model-free re-asks a friend who brought tonkatsu, no matter the box. Model-based tracks which box the tonkatsu came in, then asks the friend who usually brings that box — so tonkatsu reached by a rare swap makes it switch friends.