One marble. It is not empty after you put one marble into it.

This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town
And beats high mountain down.

Three men book a hotel room that costs £30. They pay, then go to the room. Then, the manager realises that they should have only been charged £25. He gives the porter £5, with instructions to give it back to the three men. Realising that £5 won't split between 3 people, the porter decides to give them £3 back (£1 each) and pocket £2 for himself.

The men each paid £27 pounds for the room (£30 minus £3 split between them from the refund) and the porter has £2. 27+2 = 29. Where has the missing £1 gone?

PS.
Easy-ish to work out, hard to explain the answer. DON'T google it...on your honour -_-

Situation 1:
The three men pay 30 pounds
(Status: 30 pounds manager/porter, 0 pounds men)

Situation 2:
The porter/manager gives back 3 pounds and keeps 2 pounds
(Status: 27 pounds manager/porter, 3 pounds men)

So you can not add up 27 + 2, as the 2 pounds are already added to the 25 pounds of the porter/manager, which brings them to a total of 27 pounds. And the men only received 3 pounds.

Edit, a better answer:

Let's say the men start off with 30 pounds and the hotel with 0 pounds:
The men have 3 pounds left
The men paid 27 pounds
The manager/porter has 2 (which the porter has in his pocket) + 25 (that the manager has) = 27 pounds
THe manager paid -27 (so he gained)

So you can't add up what the men paid with what the porter has in his pocket (which equals 29), as what the men paid is equal to what the manager and the porter have.

This is really confusing to explain ;)

Another riddle:

A home of wood in a wooded place, but built not by hand. High above the earthen ground, it holds it's pale white gems. What is it?

Three men book a hotel room that costs £30. They pay, then go to the room. Then, the manager realises that they should have only been charged £25. He gives the porter £5, with instructions to give it back to the three men. Realising that £5 won't split between 3 people, the porter decides to give them £3 back (£1 each) and pocket £2 for himself.

The men each paid £27 pounds for the room (£30 minus £3 split between them from the refund) and the porter has £2. 27+2 = 29. Where has the missing £1 gone?

PS.
Easy-ish to work out, hard to explain the answer. DON'T google it...on your honour -_-

£27 is the amount that the men didn't have. The £25 to the hotel and the stolen £2 from the porter (£25 + £2). £30 - £3 is their total outgoings (payment - refund). So the final sum should be £27 for what they don't have plus the £3 they do have to get the original £30.

The hotel kept £25
The porter kept £2
The men kept £3

Thing that should be added is what they kept and not what the porter stole.

What the hotel kept + what the porter stole = what the men don't have
What the men don't have + what the men have = £30

Your sum in the final paragraph was:

(What the hotel kept + what the porter stole) + what the porter stole = £29

Here's one that took me a while when I first heard it.

A man walks up to two unmarked doors, each with a doorman. There is a sign between the doors saying "One of these doors will bring you great pleasure, the other will give you great pain. One of the attendants will lie, one will tell the truth. You may ask one question for both to answer."

What question should the man ask to find out the good door?

Remember: only one question which both will answer.

Ask the liar which door will give you pleasure. He will obviously show the door of pain, lying about it being the door of pleasure. The truth teller will show you the correct door for pleasure. Choose the opposite of the liar and the door the truth teller points out.:)

Ask the liar which door will give you pleasure. He will obviously show the door of pain, lying about it being the door of pleasure. The truth teller will show you the correct door for pleasure. Choose the opposite of the liar and the door the truth teller points out.:)

You don't know which one is the liar and which is the truth teller. They would both point to different doors in your scenario and thus you would get no true answer.

Now will they? I have my doubts. Here in Belgium, we have one of the most dense road maps of the world.
Traffics jams, road constructions, pits in the road ...