In a world invaded by mobile applications, learning a second language has become easier and less expensive. With every word you learn you earn points, and with every mistake you make you lose a "heart" Duolingo is the world's leading language learning app, with 500 million downloads and profits of more than $160 million in 2020.
Apps like Dulengo and Memrise deliberately use Operant conditioning theory, which uses "incentive conditioning" to keep a child from learning like a teacher.