The sun is the ball of fire in the sky that the Earth goes round, and that gives us heat and light.

You refer to the light and heat that reach us from the sun as the sun .

If you are sunning yourself, you are sitting or lying in a place where the sun is shining on you.

A sun is any star which has planets going around it.

Everything under the sun means a very great number of things. Anything under the sun means anything at all.