Why do comets grow tails when they approach the Sun?

Comets are icy bodies that spend most of their time in the cold outer Solar System. As they near the Sun, their ices sublimate directly into gas, carrying dust with them. This produces a glowing coma around the nucleus and typically two tails: a dust tail and an ion tail. The dust tail curves because of orbital motion, while the ion tail points more directly away from the Sun due to the solar wind. Comet tails can stretch millions of kilometers and change rapidly from day to day.