If you say my name, I will no longer exist. What am I?

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

Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

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

How many letters are in the alphabet?

In an interview, a boy was asked an unusual question 'How two persons sitting with a table in between them can't see each other?' He was unable to reply. Can you?

I have a clock(12-hour format) and both the needles of the clock overlap at 12:00.
After how much time, they will overlap again?

I am beautiful, up in the sky. I am magical, yet I cannot fly. To people I bring luck, and to some people, riches. The boy at my end does whatever he wishes. What am I?

I look flat, but I am deep. Hidden realms I shelter. Lives I take, but the food I offer. At times I am beautiful. I can be calm, angry, and turbulent. I have no heart but offer pleasure as well as death. No man can own me, yet I encompass what all men must have. What am I?

Two in a corner, one in a room, zero in a house, but one in a shelter. What is it?