How can you write nineteen in a manner that if we take out one, it becomes twenty?

Look at the below sequence of letters and tell us what the next letter should be:

O T T F F S S ?

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

The first two letters signify a male, the first three letters signify a female, the first four letters signify a great, while the entire world signifies a great woman. What is the word?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

I’m light as a feather, yet the strongest person can’t hold me for five minutes. What am I?

How can one talk without saying a single word?

A man always keeps a spare tyre in his car. To make full use of all the five tyres, he changes the tyres in a manner that for a distance of 1, 00,000 km, each of them runs the same distance.

Can you calculate the distance travelled by each tyre on that journey?

You walk into a creepy house by yourself. There is no electricity, plumbing, or ventilation. Inside you notice 3 doors with numbers on them. Once you open the doors you will die a particular way. Door No.1 You’ll be eaten by a lion who is hungry. Door No.2 You’ll be stabbed to death. Door No.3 There is an electric chair waiting for you. Which door do you pick?

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?