I ride a mtb with street tires which I keep well inflated. Not overinflated, but not low pressure. Sitting watching something on tv & heard a hissing noise. At first thought it might be on the tv. No...turned off the sound to discover one of my tires going flat. Took the wheel off, checked it over very carefully. Couldn't find a thing wrong...no glass, nails, wires, sharp stones, anything. Pulled the tire off & the tube out & pumped a little air in, then stuck it into the sink filled with water to find the leak. A tiny, tiny slit, not on a seam. Carefully checked the rim, nothing sharp. So...pinch flat? Or did the tube simply fail?