Collection of brain teasers I have found from various sources:

1. There are 6 coins and a balance. All coins has same weight except for one (lighter or heaver than others) What is the minimum number of weighing required to certainly find out odd coin?

2. There are 27 balls out of which one is heavier. You have a balance. Describe the steps to find out the heavier ball in minimum number of weighing.

3. Two persons are walking into a railway tunnel. When they crossed 2/3rd of the tunnel, they saw train was coming from opposite direction. Each person ran towards a different end of tunnel with speed of 10 km per hour. Both the persons escaped from the tunnel just right time i.e. at the time train would have run through them. Assuming train was running with a constant speed and both persons reacted immediately, What is the speed of train.

4. There are thee switches outside of a closed room to operate three different light bulb inside the room. You can only go into the room once and you need to determine which switch belongs to which bulb. Just to make it clear, room is perfectly locked and its not possible to determine whether lights are on from outside the room.

5. You have 10 Jars filled with marbles. Each marble weighs 10 gm, except one Jar which contains defective marbles which weighs 9 gm. Given a scale to weigh, How do you find Jar with defective marbles in just one measurement. Here scale is modern day electronic scale to measure weight.

6 A person shoots her wife. Then holds her under water for 5 minutes. Finally, he hangs her. But after 10 minutes they both go out together and enjoy a wonderful dinner together. How can this be?

7 A father and son are going in a car and suddenly there car met accident. Father died on the spot but the child rushed to the hospital. When he arrives in the hospital, doctor says, "I can't operate on this child, he is my son!" How can this be?

8 You have a 5 liter Jug and a 3 liter Jug and unlimited supply of water. You need to measure exactly 4 liter of water but there is no measuring instrument or cup. Also Jugs are oddly shaped and doesn't contain any mark.

9 There are Four people needs to cross the bridge during night. The bridge can hold the weight of two people at a time and can not be crossed without a torch. Each person walk with different speed, first can cross the bridge in 1 minutes, second in 2 minutes, third in 5 minutes and fourth take 10 minutes to cross the bridge. If two persons go together, they walk with the speed of slower person. What is the least amount in which all four people cross the bridge.

10 A person's age is as many days as his father's age in weeks and as many month as his grand father's age in years. If you combine age of all three it comes 120 years. What is the age of all three?

11. There are two ropes which take one hour to burn from one end to other. The ropes do not burn at a constant rate (some part may burn faster). How can we measure 45 minutes by using these two ropes (obviously a lighter will be needed and assume you have it).