Much like warm air forms condensation when it interacts with a cool surface (imagine an ice filled glass on a warm day), smoke condenses when it cools down as well. The difference is that in addition to water vapor, smoke condensation (a.k.a. creosote) contains tar and soot and causes chimney fires if not cleaned out on a regular basis. Creosote builds up more quickly when you burn small, cool fires, use unseasoned wood or have a wood burning insert installed without a properly sized flue. To avoid excessive creosote build up it is recommended that you burn only well seasoned hardwood, burn hot fires, don't make a habit of burning small fires, have your chimney inspected at least once per year or more often if you use your stove as your primary heating source