What has many keys but can’t open a single lock?

Find The Next Number in this series.
1, 2, 6, 42, 1806 ?

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

A man hijacks an aeroplane transporting both passengers(8 of them) and valuable cargo. After taking the cargo, the man demands nine parachutes, puts one of them on, and jumps, leaving the other eight behind. Why did he want eight?

I am the fastest animal but cannot climb the tree. What am I?

What does below chemistry rebus means?

We have shown you a regular water barrel as below. Without using any measuring device can you check if the barrel is more than half-filled or less than half-filled?

What are the next two letters in the following series and why?
W A T N T L I T F S _ _

*Hint: Check Puzzle Title

When 99 is considered higher than 100?

In two decks of cards, what is the least amount of cards you must take to be *guaranteed* at least one four-of-a-kind?