The frequency illusion. Also known as the Baader-Meinhof Phenomenon.

You learn a new word, and suddenly you start seeing that word everywhere. It's not that it's actually occuring more frequently, it's just that you're suddenly paying more attention to it.

Earthquakes happen all the time. It's just that they're rarely as devastating as what happened in Turkey.

= is used to signify two numerical quantities are equal. 5=6-1.

≡ is used in lieu of "is the same as", verbally speaking. It denotes identities, i.e. two expressions whose value is equal no matter what the variables are. For example: (a+b)^2 ≡ a^2 + 2ab + b^2 . In most cases, we simply use = instead, but it is important to remember that it's simply an equivalence sign.

Do note that ≡ is more frequently used to denote congruence. Two numbers are congruent modulo a number N if N divides their difference. We write a ≡ b (mod n).

⇔ is "if and only if". P ⇔ Q means Q is true if and only if P is true. The truth table for this condition is such that P ⇔ Q is true in two cases: both P and Q are true, or both P and Q are false. This logical equivalence can also be written, albeit less frequently, as P ≡ Q

↔ is used exactly the same way as ⇔. No difference whatsoever.

Summary: It's better to learn these symbols as part of the required reading, there sometimes is significant overlap between what they are used for.

>Nothing is 100% transparent

I'd argue that vacuum is kapp

Because snow is just a bunch of tiny crystals that scatter all wavelengths in all directions. When you think ice, you're thinking of a solid chunk of frozen water which refracts light the same way, albeit with a lower index of refraction. If the ice happens to be cloudy because of air or impurities, it will scatter more light and appear cloudy, all the way to the point of basically becoming snow white the more impure it is.

Edit: to explain scattering in an ELI5 friendly way, ever played with marbles as a kid? Most of them used to be transparent like glass, but we used to grab a subset and rub them against a rough surface like a pavement to make them opaque. In doing so, they would take a certain color. That's basically how light scattering works, the rough uneven surface doing the work.

Meant Union Station in DC, should have probably just mentioned Grand Central since it's the train station everyone knows.

But since we can make them tall, we do. This isn't just airports. Look at Union station. Built in the 20s yet feels like a giant hall with a very tall roof. This isn't some new age architecture fad. Many will argue it's twice as important in airports to make passengers feel at ease since they're about to be crammed in a narrow metal tube for hours. Better to get a look at the blue sky and planes landing/taking off.

>Brains perfectly work rationally while horny

Completely untrue, rational thinking can be impacted by all sorts of things. You don't have to be intoxicated to have a lapse in judgment, being hungry alone is enough to make the smartest brains slow down.

As for your original question, 1) folks don't really think rationally when your brain expects pleasure, you just want it and don't have time or resources to ponder the ramifications like you would any other time, 2) people like sex better without a piece of latex getting in the way, so some deliberately choose to take the risk.