Eternally I am 1 to 6, What am I ?

Eternally I am 1 to 6,
eternally am 15 to 20,
I am always 5,
But I am never ever 21 unless I am flying,
What am I ?

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A dice :
You will find the numbers 1,2,3,4,5,6 - one on each side of a die (Eternally I am 1 to 6,)
When the die is on a flat surface, the numbers showing will add up to a number from 15 - 20.
When the die is "resting", only 5 numbers always visible.
When the die is thrown (flying), all sides are visible and they total 21.