The "Rice Knuckle Rule" Rule
Stop me if you've heard this one: how do you cook perfect rice? Well, you pour rice into a pot, touch your finger to the top of it, then pour in water till it reaches your first knuckle.
This advice doesn't make any sense. Different pots have different diameters, so it's highly unlikely that the right amount of water would always be "rice-height plus x inches" regardless of pot diameter. But that's nothing against the bigger objection: people have different lengths of fingers, so if everyone is adding water up to their first knuckle either 1) it doesn't really matter how much water you add, or 2) people are getting different outcomes with their rice, or 3) the rice-knuckle rule isn't real. Spoiler: it's the third one.
Here's how the rice-knuckle rule works in practice. Most people who cook a lot of rice are cooking the same kind of rice in the same kind of pot on the same stove, repeatedly. After the first few times of trying to rice-knuckle it, they end up developing an intuition for how much water their rice in their pot on their stove needs. Sometimes that's exactly up to the center of their knuckle, sometimes it's more like the top of their fingernail, sometimes it's almost up to the second knuckle. But everyone knows that the right amount of water is up to your first knuckle, so whatever level their water-pour actually is, they will report having poured the water to exactly one knuckle-depth. If they buy a new pot, after a couple of rice-cooks that come out too wet or too dry, they'll adjust how high they say "a knuckles-worth" is and then carry on as before.
We can call this the "Rice Knuckle Rule" Rule: what people are actually doing is following their experience and their complex personal judgement, while claiming to be following a vivid rule-of-knuckle everyone else does.
Where does this happen in everyday life? One example is the common advice to "be yourself". This is ridiculous, and clearly so. All of us have parts of our personality that we should emphasize and parts that we should work on. Many people should "be themselves" more than they are – that is, lean into their personal foibles and eccentricities and just be comfortable owning them – but "be yourself" is vague and broad and doesn't capture the truth at all. However, everyone knows that – like pouring rice-water up to your first knuckle – you're supposed to try to Be Yourself. So, people adapt their behavior to whatever actually makes sense for them, and then go around telling everyone "well, the important thing is just to be yourself!"