You are standing before two doors. One of the path leads to heaven and the other one leads to hell. There are two guardians, one by each door. You know one of them always tells the truth and the other always lies, but you don’t know who is the honest one and who is the liar. You can only ask one question to one of them in order to find the way to heaven. What is the question?

The question you should ask is "If I ask the other guard about which side leads to heaven, what would he answer?"
It should be fairly easy to see that irrespective of whom do you ask this question, you will always get an answer which leads to hell. So you can chose the other path to continue your journey to heaven.
This idea was famously used in the 1986 film Labyrinth.
Here is the explanation if it is yet not clear.
Let us assume that the left door leads to heaven.
If you ask the guard which speaks truth about which path leads to heaven, as he speaks always the truth, he would say "left". Now that the liar , when he is asked what "the other guard (truth teller) " would answer, he would definitely say "right".
Similarly, if you ask the liar about which path leads to heaven, he would say "right". As the truth teller speaks nothing but the truth, he would say "right" when he is asked what "the other guard( liar ) " would answer. So in any case, you would end up having the path to hell as an answer. So you can chose the other path as a way to heaven.

There is a low railroad bridge in your town. One day you see a large truck stopped just before the underpass. When you ask what has happened, the driver tells you that his truck is half of inch higher than the indicated height of the opening. This is the only road to his destination. What can he do to get through the underpass the easiest way?

Let enough air out of the tires to lower the truck.

While mixing sand, gravel, and cement for the foundation of a house, a worker noticed a small bird hopping along the top of the foundation wall. The bird misjudged a hop and fell down one of the holes between the blocks. The bird was down too far for anyone to reach it and the hole was too small for it to fly out of. Someone suggested using two sticks to reach down into the hole and pull the bird out, but this idea was rejected for fear it would injure the fragile bird. What would be the easiest way to get the bird out of the hole without injuring it?

Since they had plenty of sand available, they could pour a little at a time into the hole. The bird would constantly keep shifting its position so that it stood on the rising sand.

You are blindfolded and 10 coins are place in front of you on table. You are allowed to touch the coins, but can't tell which way up they are by feel. You are told that there are 5 coins head up, and 5 coins tails up but not which ones are which.
How do you make two piles of coins each with the same number of heads up?
You can flip the coins any number of times.

Make 2 piles with equal number of coins. Now, flip all the coins in one of the pile.
How this will work? lets take an example.
So initially there are 5 heads, so suppose you divide it in 2 piles.
Case:
P1 : H H T T T
P2 : H H H T T
Now when P1 will be flipped
P1 : T T H H H
P1(Heads) = P2(Heads)
Another case:
P1 : H T T T T
P2 : H H H H T
Now when P1 will be flipped
P1 : H H H H T
P1(Heads) = P2(Heads)

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.

One day a scholar came to the court of Emperor Akbar and challenged Birbal to answer his questions and thus prove that he was as clever as people said he was.
He asked Birbal: "Would you prefer to answer a hundred easy questions or just a single difficult one?"
Both the emperor and Birbal had had a difficult day and were impatient to leave.
"Ask me one difficult question," said Birbal.
"Well, then tell me," said the man, "which came first into the world, the chicken or the egg?"
"The chicken," replied Birbal, very confidently.
"How do you know?" asked the scholar, a note of triumph in his voice.
What did Birbal answer to this?

Birbal told the scholar, "We had agreed you would ask only one question and you have already asked it" and he and the emperor walked away leaving the scholar gaping.