Good Riddles

Question: Most people are born on one, die on one, and experience some of their most blissful moments upon it.

Question: I am unseen but all people know of me. I am intangible yet sharper than any sword. I come from nothingness but can fell even the mightiest kings.

Question: What can float but can't sink.

Question: What can run but can't fly?

Question: In the middle of winter, a woman is found dead in her bed, with multiple stab wounds in the stomach. Soaked into the sheets is a massive amount of blood, along with blood drops on the carpet. On the side table beside her bed, there is a glass full of water mixed with blood. What was the murder weapon, where is it now, and how was she killed?