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  "body": "\n## Summary\n\nA Turing machine is an abstract model of computation with a finite set of states and an unbounded tape of symbols.\nA moving head reads and writes the symbols one cell at a time.\nAlan Turing introduced it in 1936 to make the notion of an algorithm precise.\nA function is computable when some Turing machine computes it, and by the [[Church–Turing thesis|RWRNBS3Z7B]] this matches the informal idea of an algorithm.\nTuring machines are the standard model in [[computability theory|X14Y880PN5]] and in complexity theory.\n\n## Explanations\n\n### L1 human\n\nPicture a long strip of paper divided into cells, each holding one symbol, and a reading head that sees one cell at a time.\nThe machine is in one of finitely many states, like moods.\nAt each step it reads the symbol under the head, writes a symbol, moves one cell left or right, and changes state, all by a fixed table of rules.\nFor example, a machine can scan a word of $0$s and $1$s from left to right and flip each digit, stopping at the first blank cell.\nDespite this simplicity, every algorithm can be carried out by such a machine.\n",
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