Astronomy Fun Fact #74
Comets are balls of dust and ice that orbit the Sun. They give off tails of vaporized particles as they orbit, but those tails don't always trail behind the comet as it moves.
The Sun is constantly radiating away heat and pressure in a solar wind. When a comet gets close enough to a star, that wind can push the particles in the comet's tail away, so the tail ends up not trailing the comet itself.