Bo Derek in texbook.tex?

It's a somewhat dated joke, referencing a film that she starred in called "10".

https://en.wikipedia.org/wiki/10_(film)

Traditionally, the index in a mathematics text will contain a few funny or twisted or self referential entries.

This is one such example. Index entry "Bo Derek" points to "10" in the text. Those words appear only in the index, not in the text (but they are, of course in the TeX source). The connection is the movie referred to in @RicharSullivan 's answer.

There are other instance that should(?) have been indexed - for example

\exercise Statistics show that only 7.43 of 10 people who read this manual
actually type the |story.tex| file as recommended, but that those people
learn \TeX\ best. So why don't you join them?
\answer Laziness and/or obstinacy.

(Don't even think of trying to index all the references to 10pt .)

A google search for humorous index entry mathematics books found this from reddit:

Not a dedication, but I've seen an index entry to Gian-Carlo Rota that points to the index itself; he had no other references in the book. I think it was a real analysis text book.

One of my professors pointed it out, and the way he said it, it sounded like it was common knowledge. "Has everyone seen this by now?"

Does anyone know if this was a common thing among math text books?

................

Knuth is well known for these kinds of shenanigans.

I can't seem to document this tradition further, nor remember any examples (other than those I put in a book of my own). Perhaps visitors here can supply some (edit, or comment).

Edit. @FrankMittelbach 's comment led me to this from an interview with Knuth at https://github.com/kragen/knuth-interview-2006:

... there are lots of corny jokes in the indexes to my books now that people probably haven’t discovered yet, but somebody will ask me, why do I have a reference to Bo Derek in The TeXbook? And it turns out that just all the pages which are cited in the index for Bo Derek is where I used the number ten, so all the way through I’ve had this silly streak of some sort that means I don’t take everything too seriously.

There is just one instance of the string "Bo Derek" in the TeXbook TeX source, and that does refer to a page containing "10". I'm not sure whether it's correct to read the "just" in the quote from Knuth as claiming (falsely) that all the pages where "10" appears are indexed.