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Prof. Sema is an interactive study tool for the hardest part of an Operating Systems course: concurrency. Instead of reading about race conditions, you drive two real threads through a shared counter one operation at a time. At every step you choose the next operation from a mix that includes the one correct move and several traps — the actual mistakes students make on exams — unlabeled, so you have to judge for yourself. Pick wrong and it really executes: the bug plays out with an animation faithful to what the CPU actually did, then undoes so you can try again. Seven levels build from a no-lock race, through mutexes and semaphores (both wait()/signal() and P()/V() notation), to producer–consumer, readers–writers, a two-account transfer deadlock, and the Dining Philosophers. No accounts required — everything works anonymously with local progress; sign in with Google or Apple only if you want progress synced across devices.