What's the only room from which no one can enter or leave

Until it is measured, not known. Yet how you miss it when it has gone. What is it?

I never stop running even when I standstill. If I am formed by joining two identical bodies together, can you guess who I am?

A woman is sitting in her hotel room and hears a knock at the door. She opens the door to see a man whom she’s never met before. He says, “I’m sorry, I have made a mistake, I thought this was my room.” He then goes down the corridor and into the elevator. The woman goes back into her room and calls security. What made the woman so suspicious of the man?

A clock loses 10 minutes each hour. If the clock is set correctly at noon, what time is it when it reads 3 PM?

I look at you, you look at me, I raise my right, you raise your left. What am I?

A bomb goes off.
Carnage.
One person, only a few feet away, survives! How can this be?

It comprises roots which nobody sees,
Much taller than those trees,
Up and up it goes,
Still it never grows?

As you can see that fifteen matches have been used to form an arrangement. What you have to do is remove any six of them to make them ten.

If 21x = 79x, what is the value of x?