Multiplexers are used in computers to read and write into memory, and for a whole host of other tasks. All of the gates are examples of "switching functions", which take as input some binary-valued variables and compute some binary function. A gate is itself no more than a black box, with two inputs and one output, and a "truth table" which tells people how the output depends on the inputs. The simplest operation of all is an "identity" or "do-nothing" operation. Now with a little thought, one can see that there is a relationship between OR and AND, using NOT. A transistor is a three-connection device: one input is connected to the gate signal, one to ground, and the other to a positive voltage via a resistor. The central property of the transistor is that if the gate has a distinctly positive voltage the component conducts, but if the gate is zero or distinctly negative, it does not.