A comforter doesn’t need a duvet cover, but you can use one if you want to. Some people do this because it protects their comforter from getting dirty. A comforter can act as the insert for the duvet too.
When you crack your knuckles, it isn’t your bones that make the sounds.
The cracking sound is mainly due to the tiny bubbles that formed from the fluids of the joint. As the pressure changes, these bubbles pop, thus creating a cracking sound. In earlier days, people believed that cracking your knuckles could cause arthritis. However, modern research has shown that there is no direct correlation between the two.