Solution:

QHJRPBWF

CYLINDER = UHGQGJWA
Reverse the letters of CYLINDER
REDNILYC
Position increase in first four letters by 3.
REDN becomes UHGQ

Position decrease in last four letters by 2.
ILYC becomes GJWA.

Thus in the same manner
HYDROGEN = QHJRPBWF

Solution:

The 100th statement for sure is incorrect because it says that at least 100 of the statements are incorrect.

Suppose if that is correct, then 100 statements cannot be true.
This suggests that the 100th statement is incorrect and that the first statement is true.

Similarly 99 statements cannot be true because if they were true, then two statements would become correct i.e. the 1st and the 99th.
But the 99th statement says that at least 99 are incorrect.
This suggests that the 99th statement is incorrect and that 2ndone is true.

If we keep analyzing is the same way till the end, we will find out that only the first fifty statements are true and all the remaining ones are incorrect.

Solution:

Time

Solution:

Mountain

Solution:

This is more of a philosophical question than being a riddle or a puzzle. The action of Brian directly led to the result which is the death of Chris. Thus he murdered Chris. In a sense, Chris died due to the lack of water. It is the circumstances that ultimately led to his death.

Solution:

THINK

Change N to C
THICK

Change H to R
TRICK

Change I to A
TRACK

Change K to T
TRACT

Change C to I
TRAIT

Change T to N
TRAIN

Change T to B
BRAIN

Solution:

They can neither be true nor be false. They contradict each other and these statements are known as paradox.

Solution:

The third son bought a candle and a matchstick. He lighted the candle from the matchstick and the light filled the entire room.

Solution:

SH

Explanation :
If you look closely, you will see that the first letter of every set is moving 6 alphabets forward while the second one is moving six alphabets backwards.

Solution:

As we can catch 'Cold', 'Hot' is definitely faster.