This article is from the Puzzles FAQ, by Chris Cole email@example.com and Matthew Daly firstname.lastname@example.org with numerous contributions by others.
What exceptional words turn into other words by insertion of letters?
longest hydration (double reheadment) (D,R)EVOLUTIONIST (12/13)
longest hospitable word (insert letter anywhere)
CARES: SCARES, CHARES, CADRES, CARIES, CARETS, CARESS
shortest hostile word (no deletion possible) SYZYGY (6)
Unless noted otherwise, all words occur in Webster's Third New International
Dictionary, Merriam-Webster, Springfield, MA, 1961.