In DeFi, if you cannot name the source of your yield, you are probably the yield. Three quick checks before you act: 1. Name the mechanism in plain English: DeFi yields come from lending spreads, trading fees, protocol emissions or risk premiums. Each source has a different sustainability profile. 2. Say why it matters for behavior or portfolio decisions: That distinction is crucial because emission-driven yields can disappear overnight, while fee-driven yields tend to be more durable. 3. Set the review question: Before reacting, ask what mechanism would still matter here if the headline disappeared tomorrow. In practice: A lending protocol paying 20% APY through token emissions will see yields collapse once emissions slow. A protocol earning 4% from real lending demand has a sustainable model. Watch for: The mistake is chasing APY without decomposing where the yield originates and how long the source can persist. A lot of confusion disappears once you separate the headline from the mechanism.