In many books, easy exercises are found mixed randomly among extremely
difficult ones. This is sometimes unfortunate because readers like to
know in advance how long a problem might take -- otherwise they may
just skip over all the problems. A classic example of such a
situation is the book _Dynamic Programming_ by Richard Bellman; this
is an important, pioneering work in which a group of problems is
collected together at the end of some chapters under the heading
"Exercises and Research Problems," with extremely trivial questions
appearing in the midst of deep, unsolved problems. It is rumored that
someone once asked Dr. Bellman how to tell the exercises apart from
the research problems, and he replied, "If you can solve it, it is an
exercise; otherwise it's a research problem."
-- Donald E. Knuth
The Art Of Computer Programming