Its always 1 to 6

Always & Never
It's always 1 to 6,
it's always 15 to 20,
it's always 5,
but it's never 21,
unless it's flying.

What is it ?

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The answer is: a dice.
An explanation: "It's always 1 to 6": the numbers on the faces of the dice,
"it's always 15 to 20": the sum of the exposed faces when the dice comes to rest after being thrown,
"it's always 5": the number of exposed faces when the dice is at rest,
"but it's never 21": the sum of the exposed faces is never 21 when the dice is at rest,
"unless it's flying": the sum of all exposed faces when the dice is flying is 21 (1 + 2 + 3 + 4 + 5 + 6).