I've taken long exposures using film (analog, so no stacking or any other funny business) and saw the same thing. I always thought they were planes but now it seems they may have been satellites. I'm curious if someone knows why this happens
I've always understood "free as in beer" as: if someone hands you a beer and says it's free, you know that you don't have to pay to consume the beer, but that doesn't mean that you also get the recipe, brewing instructions, factory plans, glass making instructions etc. The only thing that is free is the liquid itself, nothing else.
Part of my PhD involved lots of fluoride synthesis using HF. I always used gloves (changing them very frequently). My advisor never used them for the following reason: if a droplet of HF lands on your glove you won't notice, but HF will go through the glove. If it lands on your bare hand chances are you'll notice it and wash them immediately.
I could never follow his advise, but I did change gloves pretty much every step in the synthesis just in case.