Read this: A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): Why does a transistor increase a signal?
the controlled (output) power can be higher than the controlling (input) power