A fluid is anything that flows, which includes gasses and liquids. Some fluids even exhibit both properties of solids and liquids, such as in response to shocks (i.e., water with corn starch, a “non-Newtonian fluid”). Ketchup is an example of a substance that does the opposite: it exhibits solid properties until it’s disturbed, after which it flows, which is why shaking or pounding on the bottom of an upturned bottle of ketchup will make it pour out.
Rigitidy is resistance to flow. To be rigid means you respond to shocks by pushing back, rather than changing shape or getting out of the way. The problem with rigidity is that while it provides the appearance of strength, failures of rigid systems and structures are catastrophic. Properly engineered structures (bridges, buildings, roads, etc.) need to be mostly rigid, but simultaneously provide enough give and take that they don’t explode when a small breeze comes along, or in the event of an extreme shock like an earthquake, avoid total collapse.
In markets and economics, we sometimes see examples of rigid systems blowing up when exposed to shocks, which sometimes result in bailouts from governments and central banks to protect from so-called “contagion” (i.e., spreading cracks can cause other rigid systems to collapse).
Some substances are incredibly rigid, to the point where they become brittle, and may shatter when stressed. Examples of this are glass or hard plastic, which–as you have likely experienced in your life–will shatter when dropped or smashed. It’s possible to harden glass or plastic, but with enough force applied, even the hardest substances will eventually shatter. These substances are still incredibly useful, but we know to carefully handle objects subject to such failures.
To be fluid means to adapt, overcome, diffuse, and take the shape of the container. Ergodicity is the mathematical term for this concept, and it has some fascinating real-life implications. In simple terms, proving ergodicity allows us to consider small samples as representative of the whole, which provides an opportunity to obtain results by sampling.
The advantage of fluidity as it relates to ergodicity and life is that you don’t necessarily need to experience the whole of anything in its entirety, a sufficiently sized sample will do. For example, you don’t need to become an Olympic athlete to find out of you’re good enough for the Olympics: you only need to play the sport a few times (or for a few seasons) to see if you’re better or worse than your peers, or to figure out if you even enjoy it.
Rigid systems will almost always blow up when exposed to unexpected or unplanned shocks. The cure for poorly designed rigid systems is to allow them to blow up, because it makes sure those systems no longer exist, and you’re only left with fluid systems. Failure is nature’s way of getting rid of the bad stuff. An often cited example of this in real life is the case of Abraham Wald and WW2 bombers, in which they chose to reinforce the parts of the planes that weren’t damaged, provided they managed to survive battles, as opposed to reinforcing the parts of the plane that had been damaged.
Fluid systems only blow up where they meet the rigid parts, which is typically around joints or interfaces that haven’t been properly designed to cope with stresses. Interestingly, the fluid parts of the system tend to help absorb shorts anyway, which makes tolerances much higher in a partially fluid system, rather than a strictly rigid system.
Rigid people struggle to adapt as the world changes around them, when they experience hardship, or when things don’t go their way. Those who are rigid expect authority to step in and save them, as opposed to learning to help themselves. Rigid people tend not to survive in a world where survival is all that counts.
People who are fluid go with the flow, they understand that nothing lasts forever, they adapt to the world around them, and they fill the container they’re in. Fluid people don’t bother fighting the trends–as much as they may not like them–and they learn to take advantage of opportunities. Those who are fluid understand that success in life is mostly about luck, but that you can increase your luck by working to maximize exposure to opportunities and making asymmetric upside bets where the downside is limited.