Why are some substances biodegradable and some non-biodegradable?

Natural or organic materials like wood, paper, and leftover food are biodegradable because their compounds can be broken down by biological processes, facilitated by microorganisms. These substances decompose into inorganic elements without leaving toxic residues.
On the other hand, substances like plastic, metals and paints cannot be broken down by biological processes and are known as non-biodegradable. Their persistence in the environment can lead to long-term pollution, as they do not break down into simpler components.