TeX "glue" (stretch and shrink)

[Schneider, Thomas (NIH/NCI) [E]]

https://en.wikipedia.org/wiki/River_(typography)

This is quite amazing to me.  When I was a kid, perhaps 5 years old
and probably before I could read, I remember seeing a page of text in
a book that had these white rivers between words.  I think my parents
were unable to see them because they were reading the words.  I never
knew until now > 10x years later that others saw them and they have a
name!

  Thomas D. Schneider, Ph.D.
  Senior Investigator
  National Institutes of Health
  National Cancer Institute
  Center for Cancer Research
  RNA Biology Laboratory
  Biological Information Theory Group
  Frederick, Maryland  21702-1201
  schneidt at mail.nih.gov
  https://alum.mit.edu/www/toms