A man dies of old age on his 25 birthday. How is this possible?

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

Solve the puzzle by telling what does below picture means.

I am not alive, but I grow; I don't have lungs, but I need air; I don't have a mouth, but water kills me. What am I?

Below, you will find the mathematical proof that 10 equals 9.99999?. But is that possible or there is something wrong about it? Can you find the error?

x = 9.999999...
10x = 99.999999...
10x - x = 90
9x = 90
x = 10

What's a single-digit number with no value?

Using exactly three matchsticks, can you divide the area into two equal parts?

What question can you never answer yes to?

You walk into an old horror house. It has no power or plumbing. Once inside, you see three doors. Each door has a number on it. Behind each door is a way for you to die. Behind door number one, you die by getting eaten by a lion. Behind door number two, you die by getting murdered. Behind door number three, you die by an electric chair. You can’t turn back, so you have to go through a door. Which door do you go through?

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211