Lawn Mower Blade

I took our lawnmower to Greive Hardware for a full tune-up, and they replaced the spark plug, changed the oil, and sharpened the blade.  I normally just remove the blade and sharpen it, like I did late last summer.  They let me know that I had reattached the blade upside down and that the mower would work much better now.  Properly installed blades lift the cut grass upwards so the mower can cut them again, hence, that's why it's called a mulching mower.  I think it's a design problem when a blade can be installed upside down; it should really just go on one way.  Lots of mower blades have something like “Grass Side” stamped on the blade, but if ours did, it's either worn off or painted over so many times it's gone.  I now know that the bent metal on both sides of the blade needs to be pointed up to be installed properly.

https://lawnmowerguru.com/which-side-of-the-lawn-mower-blade-is-up

White's Illusion

Look at the darker gray bars in column A and then at the lighter gray bars in column B.  Your brain sees them as different shades even though they are identical.  This is an example of White's Illusion, where an area's lightness is influenced by the part of the surroundings to which the area appears to belong.  In column A, the gray bars appear to be part of the long horizontal black bars; in column B, they appear to be part of the long horizontal white bars.  If you believe your eyes are different, do what I did: take pieces of paper, cover the black and white portions, and watch as they become the same shade.

https://en.wikipedia.org/wiki/White's_illusion