A cloud in a box

Hidden away
behind doors and locks
A magician kept
a cloud in a box
Once a week
he'd display his prize
to the great surprise
of questioning eyes
A young girl said
it was sad to see
and wasn't it time
to set it free?
A schoolboy declared
that it shouldn't exist
How did it get there
this fluffy white mist?
The magician shrugged
and would only reply:
that was a secret
between him and the sky