If you had a pizza with crust thickness 'a' and radius 'z', what's the volume of the pizza?

Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

Can you tell a 3D object that has just two surfaces?

On a special event at the prison, 5 prisoners John, Jacob, Krist, Tang, and Dang given a chance to go to a sauna and can bring one thing with them.

Following are the things that the prisoners bring with them.
John: A soda
Jacob: Thermos
Krist: Book named "How to kill"
Tang: A Walkman
Dang: Harmonica

Since it is a Sauna, no one can see each other. After a while When the steam is switched, Tang was found dead and there was blood all around.
J Bond was called and he instantly come to know who can be the murderer. How?

Can you make four (4) nines (9) equal 100?

Consider this about a word: The first two letters signify a male, the first three letters signify a female, the first four letters signify a great, while the entire world signifies a great woman. What is the word?

Why did the Girl throw the butter out of the window?

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

In the Chess Board picture below white army is arranged. You need to add a black army on the board such that no piece is under any threat.
Note: Army comprised of 1 king, 1 queen, 2 rooks, 2 bishops, 2 knights, and 8 pawns.

It belongs to you, but other people use it more than you do. What is it?