This is the main reason. There are tens of thousands of antibodies for different epitopes available on the market, and hundreds of dyes, enzymes etc available for detection. Making all combinations would be excessively expensive. Instead it becomes a modular system, where you can choose your primary antibody depending on epitope, and your secondary depending on how you wish to detect it.

You can stain different things in different colors at the same time by using primary antibodies from different species, effectively creating orthogonal "channels".

In some cases, however, the primary-secondary method is inappropriate. For example when doing superresolution microscopy the two antibodies on top of each other can displace the dye too far from the molecule of interest. Then you may need a conjugated primary antibody, or a even smaller single-domain antibody ("nanobody").

Smells are usually caused by chemicals right? So id imagine you get the toxins in you if you can smell them. I wouldnt worry about it tho because such small quanitites are probably not going to affect you unless you live in a house with heavy smokers

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Not all the chemicals you smell in cigarette smoke are good for you, so if the smell reaches you, so do small amounts of the not-good-for-you stuff. You didn't mention if you have multiple chemical sensitivities (MCS). People with this condition may have strong reactions to even dilute odors.

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> I understand that gravity gets less and less as you go deeper

The gravitational acceleration decreases as you descend through the core but it's still always inwards, so the center is the deepest point in the potential.

Yes, you have zero-g in the very center, but also the highest pressure.

It should only fail if there's a galvanic cell or if it's exposed to chlorine (e.g. sea water). We used to have galvanized fence wire that's easily 20 years old in the weather without any rust, but also had star pickets that were covered in rust in 12 months.

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I always assumed that they did use the pelts they had left over from kills in order to clothe themselves. The question I have is: did they discover how to create fire because they saw that their flint-napping could spark unexpected fires? Or did they discover flint napping while making fires? I'm pretty sure one would have to come with the other. Just as clothing, and subsequent survivability in colder climates probably came alongside hunting animals for food.

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Some fish have dual use lung/swimbladders, but a great many common fish have lost the respiration use entirely and the swimbladder often has no open connection at all remaining to the digestive tract. These fish often use other methods if they want to breath air, like the labryinth organ in bettas.

Even fish with totally closed swim bladders can absorb gasses from them into the blood...and push gasses into them from the blood. That's how they inflate and deflate them.

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So, it's not just a matter (no pun intended) of density but one of certain chemical affinities.

There is something called Goldschmidt classification. It's rather outdated but can describe the behaviors of elements in very broad strokes.

Essentially, it classifies elements based on which "phase" they preferentially enter. Lithophiles prefer a silicate rock phase, chalcophiles prefer a sulfide phase, siderophiles prefer a metallic phase, and atmophiles prefer a gaseous phase. In this scheme gold is considered a siderophile and would enter a metallic phase. The iron-rich metallic phase in Earth was denser than the silicate rock, so the former sank to form the core while the latter floated to form the crust and mantle. So thinking in these terms, there would be gold in Earth's core, but it wouldn't necessarily separate from the nickle-iron alloy of the core, especially since it probably only exists in trace concentrations. Of course, you'd have to determine experimentally how gold would behave under the pressure and temperature conditions present in the core.

Goldschmidt classification is nowhere near absolute, which is why we can have gold in the crust.

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That's something new for me. Thanks!

Side question though, can fish absorb oxygen with the swim bladder?