By removing one matchstick and adding one matchstick, can you convert the word "ZOO" to an animal name?

How many people must be gathered together in a room, before you can be certain that there is a greater than 50/50 chance that at least two of them have the same birthday?

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.

I never stop running even when I standstill. If I am formed by joining two identical bodies together, can you guess who I am?

Prove that the below maths equation is true.

8 + 8 = 91

What type of tree can I carry in my hand?

It comprises roots which nobody sees,
Much taller than those trees,
Up and up it goes,
Still it never grows?

He does not have any heart but still, He manages to live.
If you kill him, you will eventually die but yet you kill him.
He cannot run, who is He?

What has six faces, but does not wear makeup, has twenty-one eyes, but cannot see?

I was born in a small house and I live in it all alone. No windows and no doors have been assigned to my house. The only way I can go out is by breaking through the walls of my house.

Who am I?