Final answer:
Comet tails are formed by the repulsive force of sunlight driving particles away from the head of the comet.
Step-by-step explanation:
The tail of a comet is formed as the comet approaches the Sun. It is an extension of the comet's atmosphere and is made up of gas and dust particles. The tail always points away from the Sun due to the repulsive force of sunlight driving particles away from the head of the comet. This force is caused by the momentum carried by photons, which scatter from the dust particles and transfer some of their momentum to them, pushing them away from the Sun.