OK fizziks guys, here's a puzzle. Took this pic at a friend's house today. They have a wood-burning stove in the room. The ceiling and wall are blackened with soot where ever there is a stud. I can't think of any explanation. Some deets: - the room is a closed-in porch, probably uninsulated or at least poorly-insulated - there's a wood-burning stove just to the left of the pic. It's been in the house for decades. - those soot-markings are quite clearly coinciding with studs. They're 16 inches apart and have cross braces. Why would soot prefer to settle on the walls only where there is a stud behind it?