Hard Riddles

What’s in a forest but never grows, is in rivers but never flows, is in a valley but never blows, is in the present but never grows?

Four people need to cross a rickety bridge at night. They have one flashlight and the bridge is too dangerous to cross without it. 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 to cross the bridge?

Three prisoners are lined up single file, all facing forward. A wall separates the first prisoner from the other two. They are told that they will have either a black or a white hat placed on their head. Each prisoner can see the hats of the people in front of them, but not their own hat or the hats behind them. The third prisoner (at the back) can see the hats of the two people in front. The second person can see only the hat of the first person. The first person can’t see any hats. They are asked to identify the color of their own hat. If they guess wrong, they will be executed. If they refuse to guess, they will be imprisoned for life. How can they maximize the number of people who correctly identify their hat color?

There are 100 closed doors in a row, numbered 1 to 100. A person walks by each door, opening every door. A second person walks by each door, closing every second door (doors 2, 4, 6, …). A third person walks by, toggling every third door (closing open doors, opening closed doors). This continues for 100 people. After all 100 people have walked by, which doors are open?

Two trains started simultaneously from stations A and B and proceeded towards each other. They met at a place 120 km away from station A. After meeting, they took 9 hours and 16 hours respectively to reach their destinations. What is the distance between stations A and B?

A king has 1000 bottles of wine. A neighboring queen plots to kill him and has poisoned one of the bottles. The poison is so potent that even if diluted by a million, it would still kill the king. The poison takes 24 hours to take effect. The king has 10 servants. How can he determine which bottle is poisoned in just one day?

You’re in a prison with 2 doors. One leads to freedom, the other to execution. There are 2 guards, one by each door. One guard always tells the truth, the other always lies. You don’t know which is which. You can ask one question to one guard to determine the door to freedom. What do you ask?

In a house there are 3 light switches, all in the off position. Each controls one of 3 light bulbs in a room upstairs. You can flip the switches as many times as you want, but you can only go upstairs once to check the bulbs. How do you determine which switch controls which bulb?

You have a 3-gallon jug and a 5-gallon jug. How can you measure exactly 4 gallons of water?

A man is found dead in a circular room with no windows and no doors. There is a table and a puddle of water on the floor. How did he die?