Good Riddles

Question: My first is often at the front door.
My second is found in the cereal family.
My third is what most people want.
My whole is one of the united states.

What am I?

Question: The warden of a circular jail is extremely hyper one day so he begins running around opening cells. The jail has 100 cells numbered from 1 to 100. He runs in a circle and opens all of the cells. Next he runs around and closes every 2nd cell (starting with cell 2, 4, 6 , etc.). If a cell is open he closes it and if a cell is closed he opens it. When he finishes running by all 100 cells he opens/closes every 3rd cell (starting with 3, 6, 9, 12 etc.), then every 4th cell (starting with 4, 8, 12, etc.) and so on. He does this until he goes around and only changes the 100th cell.

When he is done what cells will be open?

Question: Why are manholes round instead of square?

Question: A mother is 21 years older than her child. In 6 years the mother will be 5 times older than her baby.

Where is the father?

Question: A teacher decides to give a pop quiz one day but all of her students refuse to take the quiz thinking that the teacher will call off the quiz. She can give only one of these students a detention for skipping the quiz. All of the students know each other's names and if a student knows he/she is getting a detention they take the quiz.

How can she threaten her students with the single detention so they all take the quiz?