Duolingo has replaced its hearts system with a new energy mechanic aimed at enhancing motivation by rewarding correct answers rather than penalizing mistakes. The new system allows users to start with 25 energy units for lessons, with the possibility of earning bonus energy for correct answers, presenting a more engaging approach to language learning while benefiting Duolingo's business model.

The upgrade email from Calendly effectively communicates the benefits of transitioning from a free to a paid plan by clearly highlighting advantages, recommending specific plans, and utilizing a reverse trial strategy that plays on loss aversion. This approach not only simplifies the decision-making process for users but also makes them feel informed and empowered about their choice to upgrade. The author notes the potential for further personalization to enhance user engagement.