What does does this rebus picture means ?

What does the below rebus puzzle mean?

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

Can you find out the number that should replace the question mark with a valid reason?

Astrology starts with the alphabet A, Biology starts with B, Psychology with P and Limnology with L.

Can you find an "ology" that begins with a P?

You are given a cube that is made with the help of 10x10x10 smaller cubes summing up to a total of 1000 smaller cubes. You are asked to take off one layer of the cubes.
How many remain now?

Below the four parts have been reorganised. The four partitions are exactly the same in both arrangements. Why is there a hole?

The more you take me, the more you leave me behind.
Who Am I?

A four-digit number (not beginning with 0) can be represented by ABCD. There is one number such that ABCD=A^B*C^D, where A^B means A raised to the B power. Can you find it?

I am taken from a mine and shut up in a wooden case, from which I am never released, and yet I am used by almost every person. What am I?