?
“Oh, stewardess? I speak Question Mark…”
Because they’re made with a slightly different process, in which the alkaline stuff bonds to the fat molecules in a slightly different way, making the resultant soap translucent instead of opaque.
WAG here.Another factor may be that most(?) bars of soap from the grocery store are whipped during manufacturing to mix air into the soap, giving them an opaque appearance.
Er, um, no, sorry, Chief, that’s a good guess but air has nothing to do with it. It has to do with what kind of fat you use, what kind of alkaline solution you use, and whether you add things like glycerine to the mix.
Here’s only one of many, many Web pages on soapmaking at home.
http://users.silverlink.net/~timer/soapinfo.html#Soap Contents
Good one DDG. Haven’t seen an Airplane reference in ages. You made my day!
Did anyone else see the blurb in Discovery magazine a month or two back about glycerine? Turns out you can inject it just under the skin of a mouse and you can see through the skin. Pretty cool.
Yeah, I saw that, and I found myself thinking, not for the first time, “Don’t these people have anything better to do?”
Zenster: 
I think lightly-pigmented skin, like a white mouse’s, is opaque or translucent because of differences in refractive index between different phases that cause light to be scattered. Glycerin, which has been used for a long time to clarify tissues for microscopic examination, evens out the refractive index differences and causes the tissue to scatter less light, which makes it more transparent.
If you look at a pile of little spheres (say, 0.1 - 0.5 mm in diameter) of perfectly clear glass, which typically has a refractive index of about 1.5, the pile will look white and opaque, even though the glass is transparent. This is because the air that surrounds the spheres has a much lower refractive index than the glass, which causes every ray of light that passes through a glass/air interface to be bent (refracted). Hundreds of random refractions cause every ray of light to be bent in a different direction, so the light gets totally scattered. If you then replace all the air with a liquid such as benzene or toluene, which both have refractive indices of about 1.5, the little pile will look almost transparent. I think when glycerine is used to clarify tissue it is doing something like what the benzene or toluene does. Clear as muddy water, right?