Solution:

Concrete floors are very hard to crack

Solution:

The total population of New York City is about nine millions. Let us assume that 1 million among them are bald which leaves us with eight millions of people that are required to have different number of hairs on their head.

Now as per the surveys, on an average, people have 100, 000 hairs on their head. Since all must have different numbers, we start with one. Let us say someone has one hair strand, someone has two, and someone has three and so on. After 100, 000 people, we are left with 7, 900, 000 people left. Since all of them are needed to have different number of hair strands, we will move up from 100, 000.

In such case, at the end, you will be having a person with 8, 000, 000 hairs which is clearly impossible (even having 1, 000, 000 hairs on someone's head is impossible).

Therefore, there must be two people who have same number of hairs on their head.

This is a classic example of the pigeonhole principle.

Solution:

This is because the queue for buying a donut is extremely long due to which the car engine becomes cool and gives trouble. While the queue for buying brownie is pretty small and thus the car gives no trouble at all.

Solution:

Just form an equilateral triangle, planting trees at the vertices. Now find the center of the triangle and plant the fourth three on that point. In this manner you will form a shape of tetrahedron and all the trees will have equal distance between each other.

Solution:

The smart answer would have been 'I will be quite glad and will take a day off from office to celebrate the happiness with my husband.'

The nature of question is hypothetical and you can answer in the same manner too. Usually the unmarried girls will be shocked at such a question but if you are smart, you can pull it off smartly.

Solution:

They used square manhole covers in Baker Street. As the side is smaller than the diagonal of the square, they were prone to fall when twisted. However the manhole covers were round shaped in Crawford Street making them impossible to fall inside.

Solution:

He fell into the Dead Sea, which lies between Israel and Jordan. The water is so salty and dense that anyone in it floats very easily.

Solution:

10 o'clock + 4 hours = 2 o'clock.

Solution:

It caused a revolution.

Solution:

No time at all it is already built