With a well-stocked toolbox of program-analysis techniques–from random fuzzing to formal verification–I’ve had the opportunity to drive many kinds of nails: traditional software, smart contracts, heterogeneous systems, and more. But in this talk, I want to focus on one particularly versatile hammer: metamorphic testing. Metamorphic testing is especially powerful when test oracles are not easily available. We’ll see it in action on program analyzers, zero-knowledge systems, and machine-learning models. This talk is not just about where metamorphic testing works, but how to make it work. I’ll confront the challenges of adapting this technique to different contexts and share my insights along the way. Whether you’re into systems, security, formal methods, or machine learning, there’s something here for you. Come see what breaks when we swing the hammer.