For Eg: If something is due on/by March 12th, March 12 is the latest you can hand it in either way. The difference is if it's due by March 12th, you can hand it in before that too. Due on March 12th means you have to hand it in on that day.
By tomorrow means by the end of the day tomorrow i.e. it includes tomorrow too. Similarly, by Friday will mean by end of the day on Friday which would include Friday as well.
“Due by [date]” normally means the thing is to be submitted— BOTH anytime before that date, AND before the relevant closing time on that final date itself.
“By today” means today is the deadline for something. Ex: “I needed to turn in my essay for my history class by today*, but I was unable to finish it in time.
Different mobs need to be dropped different distances in order to kill them. Most common hostile mobs can be killed with a 24-block drop, although witches require 30 blocks.
In the case of the hexafluorophosphate anion, a sixth electron has been donated to the phosphorus (probably by an active metal, such as Na). The anion has an octahedral geometry and some would say that the phosphorus is using d2sp3 hybrid orbitals to make single bonds with each of the six fluorine atoms.
Whales have real bones. The skeleton of a whale consists of a skull, a backbone, a rib cage, and a collection of bones that are part of the flipper, but correspond closely to the bones in the human arm and hand.