BRAIN TEASER FOR INSET IN ICT

Until I am measured, I am not known. Yet you miss me, when I have flown. What am I?

When you have me, you immediately feel like sharing me. But, if you do share me, you do not have me.

Turn me on my side and I am everything. Cut me in half and I am nothing. What am I?

A cowboy gallops into town on Monday, stays for two days, and leaves town on Monday. How is this possible?

I am an odd number. Take away a letter, and I become the same. What number am I?