Mix vinegar, sugar, and a few drops of dish soap together in a wide-mouthed container like a mason jar.
Add about a cup of warm water to the container. The water level should be about halfway to the top.
Cover the rim tightly with plastic wrap (I also used a rubber band around the rim just to be sure).
Using a sharp knife, cut a few slits in the plastic wrap.
Place the container in the kitchen or other problem areas.
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Notes
A wide-mouth mason jar works a little better than a regular one due to providing more surface area for the vinegar mixture and more surface area for holes in the plastic wrap.You can use either white vinegar or apple cider vinegar. Both will work.Be sure to create holes large enough for the gnats to easily get through. You're making slits rather than neat, round holes.