What is computation? We describe a number of related techniques that enable Turing machines to solve the halting problem. In each case, the machine’s (supposedly impossible) behaviour is made possible by a feature of the encoding scheme employed either to represent the input or to represent the output. Some means of excluding these deviant encodings is required when using the Turing-machine concept to define the traditional set of computable functions. The exercise of quarantining deviant encodings casts fresh light on Turing's historic definition of computability.