What are dead things made up of?

Answer 1

Things like wood or leather used to be a part of a living thing, so they're still made of cells.

Answer 2

Dead things are made up of the same chemical elements as living things, such as carbon, hydrogen, oxygen, nitrogen, and other trace elements. Once an organism dies, the chemical bonds that held its molecules together begin to break down through processes like decomposition, resulting in the breakdown of organic matter into simpler compounds that can be recycled back into the environment.