How many words can a pen with half refill write?

Can you identify the hidden rebus in the image below?

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

Can you solve the maths in the below-given picture equation?

I am quite hot but become too cold if you remove the first two letters. What am I?

A train full of passengers goes through a tunnel. When the train comes out from the other end, there is no single person on the train.

How did it happen?

A man escapes from jail using help from his girlfriend. The investigators suspect four girls of being the man's girlfriend.

Out of those four girls, one is his girlfriend who is lying. Two of the girls are completely innocent and are speaking the truth. One of the girls is the man's sister who is helping the girlfriend lie. Following are the statements from all four of them:

Ariana: "Myrna is his girlfriend."
Jane: "Angelina is lying."
Angelina: "Ariana is lying."
Myrna: "Jane is not his sister."

Can you find out who is the man's sister among them?

Which number should replace the question mark?

2, 3, 5, 9, 17, _ What is the next number in the sequence?

What does it mean?