You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?

What does this rebus mean?

O
N
C
E
TIME

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?

Two natural numbers have a sum of less than 100 and are greater than one.

John knows the product of the numbers and Jacob knows the sum of numbers.

The following conversation takes place between them:
John: 'I am not aware of those numbers.'
Jacob: 'I knew you wouldn't be. I am not aware myself.'
John: 'Now I know them!'
Jacob: 'Now I know them, too!'

What are the two numbers?

If you’ve got me, you want to share me; if you share me, you haven’t kept me. What am I?

In which direction should the missing arrow point?

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 Huge on Sunday and Saturday.
I am small from Tuesday to Thursday.
I do not exist on Monday and Friday.
Who am I?

If 1+9+8=1, what is 2+8+9?

What has no start, end, or centre?