Today in part II of our daytime series "Dark Shadows," we ask: "If it isn't mold, what is it, dirt or something far more sinister?" As we discussed yesterday, dark shadows unaffected by household bleach probably are not mold, but dirt. But where does it come from? The most likely culprits are kids. Other smudges are due to air movement. Dirt and dust collect on walls and ceilings around vents where heated and cooled air exits. This is normal. But if you also see dark shadows around windows or on the edge of carpet near walls, it signals air leaks and caulking is in order to seal them up. This brings us to the end of our two-part "Dark Shadows" series where we asked: "So is it dirt, mold or what?" We'll know soon. Until then stay tuned. And that's the On The House tip for today.