Changing a data structure in a slow program can work the same way an organ transplant does in a sick patient. Important classes of abstract data types such as containers, dictionaries, and priority queues, have many different but functionally equivalent data structures that implement them. Changing the data structure does not change the correctness of the program, since we presumably replace a correct implementation with a different correct implementation. However, the new implementation of the data type realizes different tradeoffs in the time to execute various operations, so the total performance can improve dramatically. Like a patient in need of a transplant, only one part might need to be replaced in order to fix the problem.