Let's unveil the magic behind Quarkus! Quarkus makes fast boot times and low memory consumption of Java applications possible thanks to a technique called compile time augmentation. Quarkus extensions are its main building blocks. They optimize applications by doing as much work as possible at compile time instead of repeating it on each boot. This may include tasks like parsing configuration files, annotation lookups, building framework metamodel, etc. We will explain all basic concepts and show some code. You will be guided through several typical problems and their solutions that occur when porting libraries and frameworks to Quarkus and GraalVM. The talk is aimed at potential authors and contributors of Quarkus extensions as well as everybody curious about Quarkus internals.