Answer:
What is weightless but it needs two person to hold it?
What has three ways out and just one way in?
You use a knife to slice my head. And weep beside me when I am dead.
Lovely and round, I shine with pale light, grown in the darkness, a lady's delight.
During the 1800's these turned women into hourglasses and is becoming more popular again these days
You use it between your head and your toes, the more it works the thinner it grows.