Why does Io have so many volcanoes?

Io has hundreds of volcanoes, many of which are active. It is one of the most volcanically active bodies in the Solar System. Io is heated up by the strong gravitational pulls of Jupiter on one side and the large moons Europa, Ganymede, and Callisto on the other. This gravitational tugging stretches and bends Io causing it to heat up, much as a ball of clay warms up as you squeeze it repeatedly. Io's surface is covered by large sulfur lava flows and irregularly shaped mountains. It is about the size of our Moon.