It's always 1 to 6
it's always 15 to 20
it's always 5
but it's never 21
and if 21, it's flying.
SOLUTION :
a dice.
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,
and if 21 it's flying : the run of all exposed faces when the dice is flying.
