Duolingo's primary engagement moat isn't language lessons — it's loss aversion tied to the 'Streak' counter. Features like Streak Freeze, Double or Nothing wagers, and widget alerts on iOS home screens trigger dopamine loops that maintain daily active usage even when learners are busy. Great case study for student product managers.

Include this sentence in your initial proposal contract: 'Project includes up to 2 rounds of design revisions. Additional revision rounds or changes to approved wireframes will be billed at ₹1,500/hour.' The moment revisions have a price tag, clients suddenly become remarkably clear about what they actually want.