Soot is the black, powdery substance left behind when something burns incompletely. When you see dark smudges on the glass door of a fireplace or black residue inside a chimney, that's soot. It forms when fuels like wood, coal, or oil don't burn completely, leaving behind tiny particles of carbon that float up with the smoke and settle on surfaces.

Soot has been part of human life since people first discovered fire. Before electric lights, soot from oil lamps and candles coated the walls and ceilings of homes. During the Industrial Revolution, factories burning coal filled city air with so much soot that buildings turned black and laundry hanging outside got dirty. Even today, if you roast marshmallows over a campfire and hold your stick too close to the flames, you might notice a layer of soot on the marshmallow's surface.

The word can also describe anything covered with this black powder. A sooty fireplace needs cleaning, and Santa Claus in old stories often appears covered in soot after climbing down chimneys.