Tetris doesn't actually hand out pieces at random. Early versions used pure randomness, so a given piece could fail to appear for dozens of drops in a row. To fix this drought problem, later games switched to a '7-bag' system that guarantees all seven pieces appear once before repeats.

Tetris pieces are not completely random. Modern versions group the seven types of pieces into a 'bag' and randomize only their order within it. Once all seven pieces have appeared once, the next bag follows.