Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

Monty Hall problem

A game-show puzzle where switching doors after a non-random reveal doubles the win probability from 1/3 to 2/3.

Switching wins with probability $2/3$ ; staying wins with probability $1/3$ .