No, not Einstein's theory of relativity. Rather, the fact that so many things we do and think about, we do by referring to their relation to other things.
Let's start with an easy one. Sitting with Sweetie in our church's little memorial "garden," really it's a couple of benches in the wooded area that is popular for scattering loved ones' ashes, we both noticed some brand-new oak leaves, knocked out of the trees by the previous evening's hail storm. I had a tiny leaf in my fingers, slowly twirling it, admiring its faint layer of fuzz and the delicate shade of newly-minted green. Sweetie took it from me and examined it.
"You know what this means, don't you?" Something about the leaf had been gnawing at me, and suddenly I remembered what it was - not just the leaf, but the fact that it was a tiny, new oak leaf.
"Time to plant corn."
Sweetie nodded. "When the oak leaves are the size of a squirrel's ear, it's time to plant corn." That's an old standby and as rules of thumb go, it's pretty reliable. By the time such leaves are that kind of size, it's a fair chance that all your bad weather is behind you. Get out the plow and lube up the seed drill, time to get some money in the ground.
Ooh, "Rule of thumb" is a good one, too. There are different versions of the origin of the phrase, the most well-known and most quoted probably being that a man couldn't beat his wife with a stick thicker than his own thumb. That's a pretty misogynistic thing to go bandying about and I think it's likely crap. The thumb/stick relation is a misrepresentation of an old British common law that had nothing to do with anyone striking anyone, with a stick or anything else. And there are other representations of things called "rules of thumb" that go back even farther in history than that.
In fact, in several languages, the word meaning "inch" is very similar to, or the same as the word meaning "thumb." If you didn't have a ruler handy, you probably at least had a thumb. Mark off your measurements with that, extrapolate, and off you go with your project, with everything measured to the same basic unit. What difference does it make if you have huge thumbs or little ones?
I used to know a guy whose last name was - and probably still is, though I haven't seen him since high school - Smoot. It turns out that all the Smoots in the US are probably related to each other, including to the one who, in a highbrow frat stunt, was turned end-over-end across the length of Harvard Bridge in 1958, to determine the bridge's length at 364.4 smoots. Amusingly, when the bridge was renovated in the 80s, the concrete of the new sidewalks was scored at one-smoot intervals instead of the more traditional six feet. So if you think the concrete sections on the Harvard Bridge are only 5'7" you're right.
You can also find distance in smoots on Google Earth. I tried to paste an image of a screenshot but my kung fu is on the fritz.
Hmm, what else? I do love the metric system: a meter is one hundred centimeters; one thousand cubic centimeters is exactly a liter, which (of water) weighs exactly a kilogram. All the different measurements refer to each other, so if you can establish one you can find all the others with simple math. But what determined the original meter? It's not related to the "imperial" system at all. In fact, the meter is based on time.
You heard that right: time. It makes sense when you think about it, too. We talk about music being played at this or that meter, and a meter's length was originally defined as being that length of a pendulum whose half-period was one second. But certain vagaries come into play: a meter is slightly longer or shorter, depending on where you stand. Slight differences in Earth's gravitational field make the period longer or shorter according to where you are when you measure.
And of course, there are certain difficulties in measuring the exact duration of a second. It used to be defined as 1/86400 of a day, but that was hundreds of years ago. Days are longer now. Now you have to refer to other things to determine the length of a second.
Heisenberg was right. You can't observe anything without affecting the phenomena you observe. We observed the second, and depending on where you stand, it may be longer or shorter. And by golly, that affects the length of the meter! Which affects the weight of the kilo, which means pounds are bigger, which means more butter on my toast...
It's all relative. Step out of your car this morning on your way to work. Think about your shoe size, and barley. They're related, believe it.