How you know you paid too much for that $75 textbook.
"Not only was this in a textbook, but my professor is the author."
This is how med students do the shocker... from a textbook.
Just a guess, but the author of this immunology textbook has tried and failed to have a threesome.
Weird: This is from a Grad school textbook. Weirder: It is a physics textbook.
"Oh, how I love Physics Textbook diagrams."