logic You are walking down a path when you come to two doors. Opening one of the doors will lead you to a life of prosperity and happiness, while opening the other door will lead to a life of misery and sorrow. You don't know which door leads to which life.
In front of the doors are two twin brothers who know which door leads where. One of the brothers always lies, and the other always tells the truth. You don't know which brother is the liar and which is the truth-teller.
You are allowed to ask one single question to one of the brothers (not both) to figure out which door to open.
What question should you ask?

Ask "If I asked your brother what the good door is, what would he say?"
If you ask the truth-telling brother, he will point to the bad door, because this is what the lying brother would point to.
Alternatively, if you ask the lying brother, he will also point to the bad door, because this is NOT what the truth-telling brother would point to.
So whichever door is pointed to, you should go through the other one.

## Similar riddles

See also best riddles or new riddles.

cleanlogicThree people check into a hotel. They pay $30 to the manager and go to their room. The manager finds out that the room rate is $25 and gives $5 to the bellboy to return. On the way to the room, the bellboy reasons that $5 would be difficult to share among three people, so he pockets $2 and gives $1 to each person. Now, each person paid $10 and got back $1. So they paid $9 each, totalling $27. The bellboy has $2, totalling $29. )
Where is the remaining dollar?

Each person paid $9, totalling $27. The manager has $25 and the bellboy has $2. The bellboy's $2 should be added to the manager's $25 or substracted from the tenant's $27, not added to the tenants' $27.

logicYou have two jugs, one that holds exactly 3 gallons, and one that holds exactly 5 gallons. Using just these two jugs and a fire hose, how can you measure out exactly 4 gallons of water?

Fill the 5-gallon jug to the top, and then pour it into the 3-gallon jug until the 3-gallon jug is full. You now have 2 gallons remaining in the 5-gallon jug. Pour out the 3-gallon jug, and then pour the 2 gallons from the 5-gallon jug into the 3-gallon jug. Finally, fill the 5-gallon jug to the top and pour it into the 3-gallon jug until it's full. Since there was only space left for 1 more gallon in the 3-gallon jug, you now have exactly 4 gallons in the 5-gallon jug.

cleanlogicshortwhat am II am a mother and a father, but have never given birth. I'm rarely still, but I never wander. What am I?

Tree.

logicmathshortA man gave one son 10 cents and another son was given 15 cents. What time is it?

1:45. The man gave away a total of 25 cents. He divided it between two people. Therefore, he gave a quarter to two.

logicmathHow can you divide a pizza into 8 equal slices using only 3 straight cuts?

Cut 1: Cut the pizza straight down the middle into two halves.
Cut 2: Keeping the two halves in the place, cut the pizza straight down the middle at right angles to the first cut (you will be left with 4 equal quarters)
Cut 3: Pile the 4 quarters on top of each other and cut through the middle of the pile. You will be left with 8 equal slices.

logicThere are 3 switches outside of a room, all in the 'off' setting. One of them controls a lightbulb inside the room, the other two do nothing.
You cannot see into the room, and once you open the door to the room, you cannot flip any of the switches any more.
Before going into the room, how would you flip the switches in order to be able to tell which switch controls the light bulb?

Flip the first switch and keep it flipped for five minutes. Then unflip it, and flip the second switch. Go into the room. If the lightbulb is off but warm, the first switch controls it. If the light is on, the second switch controls it. If the light is off and cool, the third switch controls it.

interviewlogicmath Four people need to cross a rickety bridge at night. Unfortunately, they have only one torch and the bridge is too dangerous to cross without one. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the bridge?

It is 17 mins.
1 and 2 go first, then 1 comes back. Then 7 and 10 go and 2 comes back. Then 1 and 2 go again, it makes a total of 17 minutes.

cleanlogicwhat am IWe are a family of 12 members. I am the second. I am also the youngest in our family. Who am I?

February. A year has 12 months and February is the second month.

cleanlogicshortWhen you put this in a heavy wooden box, the box will become lighter.

A hole.

logic In medieval England, a king's jester was imprisoned (the king didn't like the jester's jokes). The jester was locked in a room at the top of a high tower. The room had only one tiny window. The jester found a piece of rope. It wasn't long enough to reach the ground. So, he divided it in half and tied the two halves together. This made the rope long enough and he escaped. How?

He divided the rope vertically, not horizontally.