Answer: Lawn mower blades can become dull when you consistently run over rocks and debris. The resulting nicks can cause the blade to tear rather than cut the grass blades. You can tell your lawnmower blade needs sharpening if after cutting the grass the tips of the blades turn yellow (because they have been ripped rather than cut). Most lawn mower blades can be removed and filed down. One with a square rather than a beveled edge should probably be replaced or sharpened professionally.
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