Comets
Comets are balls of ice, dust, and rock that grow bright tails when they travel close to the Sun.
Think of it this way
A dusty snowball sits at the far end of a cold kitchen tray. Near one end of the tray, a heat lamp glows bright and warm. As the snowball rolls close to the lamp, ice on its surface turns to steam and blows dust off behind it. That trailing cloud of dust and vapor streams away from the lamp like a long, bright tail.
Explaining comets by grade level
You can build a tiny comet using dry ice, dirt, and water. The dry ice is so cold it smokes and fizzes. That is like what happens to a real comet near the Sun. The Sun warms the ice, and gas shoots off in long tails.