What has many keys but can’t open a single lock?

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

I sleep during the day and fly at night, but I have no feathers to aid my flight. What am I?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

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 a number of books on a shelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?

The dirtier I get, the whiter I look. Who am I?

Three ants are going up a hill, one behind the other. The last ant then says to the other ants, 'There is an ant behind me!'' How come?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

John enters a small room. The Door Closes.
When the door opens, John is in a larger room.

Explain?