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

I shave every day, but my beard stays the same. What am I?

You need to complete the maze by entering from the entrance marked below in the figure near the yellow circle, bottom left and leaving from the exit point near the green circle, bottom middle.

Rule of Game: You can move only by exchanging green and yellow circles.

A chess tournament is taking place on knock-out terms (the one who loses the match is out of the game).
(a) If 10 matches are played in total, how many players participated?
(b) If 20 players took part in the tournament, how many matches were played?

It can't be seen, can't be felt, can't be heard, and can't be smelt.
It lies behind stars and under hills, And empty holes it fills.
It comes first and follows after, Ends life, and kills laughter.
What is it?

What 4-letter word can be written forward, backward or upside down, and can still be read from left to right?

When I asked a girl's name she told me her name is the date '07/09/2001'.

I thought for a few seconds and then I got her name. Do you?

I make two people out of one. What am I?

The centre of the third figure is empty. What digit should fit inside?

Can you start the fire using two pieces of wood?