You�re standing for a door, behind that door is a room with only one lightbulb.
There are 3 switches next to the door (on the outside of the room).
You have to find out which one is the right one to turn on the light.
Sounds easy, huh?
This is the catch: you may only enter the room once to check the light.
How can you find out which switch is the right one?

Remarks:
It�s impossible to look in the room, there are no gaps.
Once you go inside the room you can�t use the switches anymore, you go outside and close the door behind you.
You may use the switches as often as you like and whenever you want.

The Riddle of the gates

In front of you are two gates, one of them is the right gate to go through.
There are two guards: one always lies and one always tells the truth.
You may ask only one similar question to both of them to find out which is the right gate.
You don't know which of them lies or tells the truth.

What'll be your question?

The bridge and the bomb

Four people are standing at one side of a bridge (A, B, C and D).
They have to cross the bridge cause the bridge will be blown up in exacly one hour. It's night and they have only one electric torch, it's impossible to cross the bridge without a torch.
Their is more to it:
The maximum number of people the bridge can hold is two and they're not all four as fast:

A can cross the bridge in 5 minutes
B can cross the bridge in 10 minutes
C can cross the bridge in 20 minutes
D can cross the bridge in 25 minutes

How should they handle this?

The long fuses

You have a whole bunch of fuses (slow matches) and they all burn exactly one hour after litting one of the ends.
The fuses don't burn the same speed during the process, for example: it's possible the first half of a fuse burns in 10 minutes and the other half burns in 50 minutes.

How can you measure 45 minutes exactly with these fuses?