How rusty is your 8th grade science?

Why are there so many things out there comparing adult knowledge to that of an 8th grader? What's up with that? Is there something special about the 8th grade that I don't know about (possibly because I am not smarter than an eighth grader)?

I couldn't resist giving this one a whirl:

Dang. A- is OK I guess, but with two masters degrees and many years of postgraduate education in physics, I shouldn't have missed any of these. In my defense, the two I missed had nothing to do with physics.