Can anyone tell me a word in the English dictionary which has a silent 'z'?

I am white when I am dirty, and black when I am clean. What am I?

What does this simple rebus mean?

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?

Three brothers Jacob, John, and James live in Mexico City. The product of the ages of these brothers is 175. Jacob and John are twins. How old is James?

John is the son of Jacob.
So,
Jacob is the_____of John's father?

What is the word or phrase?

Luis Garavito killed Density x Volume.

Who is Luis Garavito ?

Irene killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

John: "Hi Teacher, What is your favourite movie ?".
Chemistry Teacher: "Indium Cerium Platinum Iodine Oxygen Nitrogen".
Ten seconds later... John replied I got it.

Do you?