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I see what you're saying. If you wanted to be as precise as possible you could calculate the mass of displaced air and add it to the weight of the sponge.

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The buoyancy force would reduce the apparent mass of the cup by the mass of the displaced air. It may not be significant but if you want to be precise, you need to account for it.

Just like how a helium balloon isn't negative mass, it just appears to be because of the buoyancy force, it's important to account for the mass of the displaced air to be precise.

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Where I live they find out the fetus’s blood type and check for mismatch. There is enough fetal dna floating around in the mother’s blood to check things like blood type, gender, and chromosomal abnormalities (like Down’s syndrome).

This is kind of the opposite of what usually happens. During pregnancy, fetal red cells can cross the placenta causing an immune reaction in Mom. The antibodies from this reaction cross the placenta and can attack fetal red cells. There is a spectrum of severity ranging from hydrops fetalis to clinically insignificant. The antibodies linger in fetal circulation after birth and subsequent red cell destruction can contribute to jaundice. You can test for the presence of these antibodies with a Coombs test.

Well blood types A, B, and AB produce mainly IgM antibodies which cannot cross the placenta so this is really only an issue when the mother is blood type O, which produces mostly IgG antibodies that can in fact cross the placenta. However I think the percentage of developing HDN is very low, something like 4%, and when it happens it is mild. Something about the baby’s antigens not being properly developed so the antibodies cannot properly bind to destroy the blood cells.

It depends on which density you want: bulk density or density of the solid fraction only, and of course how precisely you want to determine the value.

With solids or liquids, density is easy to determine, all you need to do is establish the mass (weight), which you use with a scale, and volume, which you can typically get by adding the material to a known volume of liquid in a measuring device like a graduated cylinder, and measure the volume change after addition. You could actually do this at the same time, by having the volumetric container on a scale when you add the liquid or solid. Get both volume and weight change in one move.

This will work with a sponge too, to get the density of the sponge material (assuming that you have total permeability, no blocked void spaces when immersed in liquid).

If you want a bulk density of something porous like a sponge, the best method is to measure mass (weight) dry and wet (saturated). The volume of the open space is defined by the mass of water contained within that open space (the change in mass from dry to wet is due to contained water only, and we have established water density to more precision than you will likely need for your purposes-you probably won't need to correct for temperature).

I don't do gas measurements, which generally has a pressure dependence and you would need to do a pressure-mass curve using a fixed volume container (a glass bulb of known volume, seems a good way to go). Empty the glass bulb of all air, measure its weight, then inject it with the target gas, and measure its weight. The change will be due solely to the added gas mass. Do that at a few different pressures to establish the pressure-density curve.

You could, conceivably, crush the porous solid to eliminate pore space and then get the powder mass and volume the same as with any solid. From that, you could establish the pore volume in the original sample (measure volume of the uncrushed sample and its weight and go from there with some simple math).

If you want to determine surface area of a porous solid, well, then you would need to go into more complicated or higher tech methods.

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I've seen some other stuff recently that suggests the other inner planets like Mars and Mercury have slightly unexpected composition, like Earth and the moon. This has led to speculation that there was potentially a "billiard ball era" where chunks of protoplanets were smashing into each other, distributing heavier elements more evenly across the inner planets, and explaining the traces here and there that don't line up with Theia. A rogue body like a pluto- sized comet passing through and impacting one protoplanetary mass could lead to a whole series of impacts. There's still a ton of theories about the asteroid belt, and a bunch of new studies on the composition of the larger bodies in it. Exciting stuff!

The moon is actually really, really close in cosmic terms for a moon of its size relative to earth. Saturn and Jupiter's moons are much smaller AND much farther away. and it is in fact slowing down veeeeeeeeery marginally over time, so if the sun didn't explode eventually it might make it back to earth

I'm glad your took it that way after my explanation, in opposite to the last sentence of your initial response which I found a bit agressive :)

The air is neutrally buoyant. It's not that it contributes a small amount to the weight, it contributes zero. For example if you had a cup filled with nothing but air and weighed it the air won't affect your reading at all because the weight of that air is 0 when you're in Earth's atmosphere.

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All Rh- pregnant women are given RHIg (WinRho, Rhogam etc) to minimize the chance that there is Rh incompatibility. While it is possible to assume that there will be no issues when both parents are Rh-, it is generally accepted practice to immunize the mother just the same. This page from Cleveland Clinic has a good explanation of the issue:

https://my.clevelandclinic.org/health/diseases/21053-rh-factor