What has no start, end, or centre?

Why do people go to bed?

When you stop to look, you can always see me. But if you try to touch me, you can never feel me. Although you walk towards me, I remain the same distance from you. What am I?

Who always enjoys poor health?

As easy as you can get into me, it is as hard to get out of me.

Do you know what I am?

ENEMY = 2 * LINK
LINK + 2 = HELL

Then what will be TAXI + HELL? Select from the options below:
a) ROTTEN
b) DANGER
c) HEAVEN
4) MORTAL

They came out at night, without being called.
They lost at day, without being stolen.
Who are they?

What is full of holes but still holds water?

What jumps when it walks and sits when it stands?

Replace the question mark with the correct number in the below-given picture?