What does this simple rebus mean?

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

I have two sand hourglasses:
1. A 7 minute one and
2. An 11-minute one.

Using just these 2 sand hourglasses, how can I measure time as 15 minutes?

Cristina leaves home and then she makes exactly 3 left turns and returns home where she found 2 girls wearing the mask.

Who are these two masked girls?

I guess 76.8% of readers will like this riddle.

What's the least number of people should read this post?

Name a key which is hardest to turn.

After teaching his class all about Roman numerals (X = 10, IX=9 and so on) the teacher asked his class to draw a single continuous line and turn IX into 6. The teacher's only stipulation was that the pen could not be lifted from the paper until the line was complete.

An evil man kidnapped someone and made them take one of two pills. One was harmless, but the other was poisonous. Whichever pill the victim took, the kidnapper took the other one. The victim took their pill with water and died. The kidnapper survived. How did the kidnapper get the harmless pill?

You can take four of the five letters out of this word, but the pronunciation never changes. What is the word?

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?