My cousin Scarlett has some pale pink nail polish that is exactly the same color as my actual nails*, so I painted two of them on my left hand. If I look at just the polished two, I can tell that they're polished. If I look at just the unpolished nails, I can tell that they're unpolished. But if I just look down at my hand, it is unclear which is which. It is my favorite sort of sneaky illusion - the kind that is quite difficult, that makes no difference, and that doesn't impress anyone because they can't even tell you're doing it.
*relevant: I have played piano since age 7 and don't let my nails grow long enough to have a white part.
*relevant: I have played piano since age 7 and don't let my nails grow long enough to have a white part.