Tiny little fibers can get caught in the little hole that the optical light shines from ...they are very hard to see, and you need a magnifier sometimes. My mouse gets these because of my cats, and the screen will jump when there are fibers caught in there.
I just use a tweezers to pluck them out.
I've never had a spill at the computer yet... I don't put liquids near it ever, with the cats walking around like they do!
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All truths are easy to understand once they are discovered; the point is to discover them.-- Galileo Galilei
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Weezie looking at petunias 8.25.2017
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