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Astronomers have spotted thousands of comets in the solar system, but how did they form?

See how comets grow from microscopic particles to objects the size of mountains in this video.

Particles of dust coated with water ice and other molecules serve as a comet's building blocks.

Dust particles combine to form icy rocks that join together under the force of gravity.

High-speed collisions result in a loosely bound collection of fused rock and ice.

When a comet passes through the inner solar system, its ice warms, releasing a trail of gas and dust.