what does this picture(rebus) riddle means ?

No matter how little or how much you use me, you change me every month. What am I?

A mules travels the same distance daily.
I noticed that two of his legs travels 10km and the remaining two travels 12km.
Obviously two mules legs cannot be a 2km ahead of the other 2.

The mules is perfectly normal. So how come this be true ?

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

I am beautiful, up in the sky. I am magical, yet I cannot fly. To people I bring luck, and to some people, riches. The boy at my end does whatever he wishes. What am I?

Why is the Hole below a Lock?

One absent-minded ancient philosopher forgot to wind up his only clock in the house. He had no radio, TV, telephone, internet, or any other means for telling time. So he travelled on foot to his friend's place a few miles down the straight desert road. He stayed at his friend's house for the night and when he came back home, he knew how to set his clock. How did he know?

Can you solve the photo plexer logic ?

I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I?

I have no life, but I can die. What am I?