Purely functional data structures stored inside a mutable variable provide an excellent concurrent data structure—obviously correct, cheap to create, and supporting snapshots. They are not, however, scalable. We provide a way to retain the benefits of these pure-in-a-box data structures while dynamically converting to a more scalable lock-free data structure under contention. Our solution scales to any pair of pure and lock-free container types, while retaining lock-freedom. We demonstrate the principle in action on two very different platforms: first in the Glasgow Haskell Compiler—extending GHC to support lock-free data structures and introducing a new approach for safe CAS in a lazy language—and second in Java.