I have teeth but i don't bite. Who Am I ?

There was once a college that offered a class on probability applied to the real world. The class was relatively easy, but there was a catch. There were no homework assignments or tests, but there was a final exam that would have only one question on it. When everyone received the test paper it was a blank sheet of paper with a solitary question on it: 'What is the risk?'.Most students were able to pass, but only one student received 100% for the class! Even stranger was that he only wrote down one word!
What did he write?

What number when divided in half becomes zero?

All of you like to eat me. I am a five-lettered word. Remove the first two and I turn into an infamous animal. Remove just the first and I become a heinous crime. Remove the first and the last and you can groove on me.

Can you guess who am I?

How can you throw a ball as hard as you can and have it come back to you even if it doesn't hit anything there is nothing attached to it and no one else catches or throws it.

A man dies of old age on his 25 birthday. How is this possible?

I have a hole in my back and legs I lack.
I live where I can’t breathe and I eat without teeth.
What am I?

What can you serve, but never eat?

What goes up but never comes down?

using four eights (8) and a one (1) and one mathematical symbol , create the number 100