It is shown that any finite list of smooth, closed, simply-connected 4-manifolds that are homeomorphic to a given one X can be obtained by removing a single compact contractible submanifold (or cork) from X, and then regluing it by powers of a boundary diffeomorphism. Furthermore, by allowing the cork to be noncompact, the collection of all the smooth manifolds homeomorphic to X can be obtained in this way. The existence of a universal noncompact cork is also established. † If X C,h and X are diffeomorphic, the embedding C ⊂ X is said to be trivial. The cork (C, h) is trivial if all its embeddings in all 4-manifolds are trivial, or equivalently if h extends to a diffeomorphism of C [4].