What has one letter and starts with an E?

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?

I have four wings but can't fly.
I express no emotions.
I am found on the exact spot every time you look.
I keep toiling away with a little sound.

Do you know what am I?

I have an eye but still cannot see.

Who am I?

When I go in I might cause pain. I cause you to spit and ask you not to swallow. I can fill your hole. What am I?

I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter. What am I?

A new Engineering building containing 100 offices had just been completed. John was hired to paint the numbers 1 to 100 on the doors. How many times will John have to paint the number nine (9)?

You knock a Table over in the library, and suddenly 20 eyes are looking at you.

If eleven plus two equals one, what does nine plus five equal?

Before Mount Everest was discovered, what was the highest mountain in the world?