What work can one never finish?

Two friends were betting. One said to the other, "The coin will be flipped twenty times and each time the coin lands on the head, I will give you $2 and each time it lands on the tale, you will give me $3." After flipping the coin twenty times not a single penny was exchanged among them.

How many times did the coin land on heads?

I know a ten-letter word in the English language which can be typed using only the top rows of the computer keyboard.

I look at you, you look at me, I raise my right, you raise your left. What am I?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

Two boys were admitted to a school. When the headmaster asks them about their parents, they tell him that they have same parents (father and mother). On further inquiry, it turns out that they both share the same date for their birthday.

"Are you twins," ask the headmaster.
"No," replies the boys.

Is it possible?

What jumps when it walks and sits when it stands?

Here is a mathematical expression that also secretly tells a movie name. What movie is that?

The teacher told the student that if he told a lie then he will be expelled from school and if he told the truth then he still is expelled from school.

What can a student say to prevent his being expelled from school?

What is the centre of gravity?