Turing machine
When a Turing machine is started, a word (out of the working-alphabet) must be present on the infinite tape of the machine. The read/write-device on the first character now reads the first character and depending on the current state of Turing machine the read/write-device overwrites the character with a new one or moves one cell to the left or to the right. Furthermore, the current state of the machine can be switched.