It has no color, but you can see it.
It weighs nothing but everything gets lighter through it.
It is completely silent, but often it's needed to hear something (better).
It doesn't send out light, but often it's needed to see something.
One can create it but cannot buy it.

Air.

Air has no colour - how would something get lighter?

The only problem is that it doesn't really follow this rule: "often it's needed to see something."
It fits every other property.

You don't need air to see something.

Next hint:
It devaluates many things but many things would be without value without it.

edit

How about... logic?

Logic doesn't make things lighter.

Next hint: (the last official one. If it's not enough I can either reveal the solution or make up more - less interesting - hints. You decide.)
One creates it by taking something away, but removes it by adding something.

Keep in mind that I said Space/The endless void was close to the solution.

A Vacuum.

*checks thread for first time in a few days*

A hole.

right, your time

Suppose you want to send in the mail a valuable object to a friend. You have a box which is big enough to hold the object. The box has a locking ring which is large enough to have a lock attached and you have several locks with keys. However, your friend does not have the key to any lock that you have. You cannot send the key in an unlocked box since it may be stolen or copied.

How do you send the valuable object, locked, to your friend - so it may be opened by your friend?

Leave the key inside the wrapping paper but outside the box?

You send the package with any lock,
You send a locked box with the key for the package.
And you send the key for this locked box in an unlocked envelope.

No. Unlocked envelope would essentially be the same as sending it in an unlocked box. Same goes for you KK.

Don't lock the box at all?

_{This probably isn't right, but I just wanted to make sure it wasn't a trick question >>}

Yeah... no.

Really simple, actually.

Send the box with a single lock to your friend, then have him attach a lock of his own, and send it back. Then remove your own lock, and send him back the box with his own lock on it.

Post has been edited 1 time(s), last time on Nov 30 2008, 8:52 pm by Greo.

It specified 'a' lock.

Then why would he bother to say you have several locks with keys? If all else fails, you could just chain the box up and place a lock on that.

Anyway, here's the next one.
A tennis ball fell into a hole nearby, while you were playing tennis. The hole happens to be just big enough for the ball to fall in easily, but not big enough for your hand to reach in. Using only things on hand, what can you do to retrieve the ball? (In case you were wondering, the hole is about 30cm deep.)

Greo wins.

I hope he didn't just look it up online.