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.
The person who took this picture didn't even read it.
This is how med students do the shocker... from a textbook.
"Not only was this in a textbook, but my professor is the author."
Now you're scared enough to stay up all night and study.