Here's the URL to a very, very short Grimm's tale -

http://www.cs.cmu.edu/~spok/grimmtmp/209.txt

It's called the Golden Key. The key in the story is higher knowledge.
The boy has found a template, so to speak. Higher knowledge and
practices. Practical knowledge to use to develop conscious, inner
development. (Also, that the boy is wishing to 'start a fire' is
symbolic of what you need to do when you are 'in the work'.

In the Manheim translation it is stated more clearly that the boy is
'turning the key'... When he finishes turning the key we will know
what wonderful things are in the box. The 'turning' is the effort
required.

Some people want to know what is in the box before they make the
effort to turn the key. You've found the key, now TURN the key, this
tale is saying. Make the effort (practice the work in real time, real
life). Once that is done we will find out what wonderful things are
in the box... (The work gives you hints, but you have to turn the key
yourself to find out.) - anon.

ps- This seems like such a simple interpretation and an obvious
statement, yet the 'wanting to know what is in the box before any
effort is made' is something that appears in so many people's
approaches to this work in different ways. For instance, some of the
voices on the various fourth way internet forums that have, in various
ways, suggested that the ideas and practices and goals of the work (to
the extent that they understood them, which is probably very little)
are all about self-deception and imagination and escape or whatever;
these voices are not just necessarily demonstrating a lack of
valuation for the work but are also expressing, in some cases, a
disappointment that they can't know what's in the box without making
real efforts to turn the key...