Recall that a problem is in **co-NP** if its complement is in **NP**. Start by identifying the defining characteristics of **NP** problems (verifiable solutions in polynomial time) and then invert them to understand **co-NP**.

Consider **NP-complete** problems (e.g., SAT) and their complements. If a problem is **NP-complete**, its complement is **co-NP-complete**. Use this to derive concrete examples like **Unsatisfiability** (complement of SAT) as a **co-NP** problem.

Explore **graph-theoretic** problems: **Graph Non-Isomorphism** (complement of Graph Isomorphism) is a candidate for **co-NP**, but its exact classification remains unresolved. Discuss why proving it lies in **co-NP** (or not) is non-trivial and relates to major open questions in complexity theory.