Creosote occurs when smoke from an open flame travels up your chimney and mixes with cold air and water. The substance then solidifies to the chimney liner or bricks and over time will build up and become dangerous. While creosote is a natural byproduct of burning wood, it is extremely flammable and can catch fire if you let it build up too much. A surefire way to prevent a chimney fire (or even worse, a house fire) is to have your chimney cleaned frequently.