A healthy starter smells pleasantly sour, like yogurt with a hint of apple or grain vinegar, and the exact note shifts with the flour and the temperature. At peak it smells balanced; after a day unfed it sharpens toward nail-polish acetone as alcohol builds.
Acetone and a grey liquid layer mean a hungry starter, not a dead one: pour off the liquid, feed fresh, and it returns in a day. A cheesy, sock-like smell often means it went too warm and bacterial; a few days of cool, consistent feeding usually resets it.
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Only paint thinner, putrid, or mold-colored smells, with pink or orange streaks, mean discard. Everything else is recoverable with regular feeding at 100 percent hydration.