Overlaps with one's interlocutor in the choice of words are called lexical entrainment. This article looks at accounts for these overlaps in word use. The question addressed is the extent to which the word use of the addressee, as opposed to available words from other sources, has a special impact on experts' choice of words. A laboratory experiment was conducted that addresses this question in the domain of online health advice. Eighty advanced medical students responded to a fictitious patient inquiry via e-mail. Results indicate that the mere availability of words (whether they were used by the addressee or not) accounts for experts' word use. Nonetheless, an analysis of experts' reflections on the patient's knowledge also indicated a metacognitive awareness of the specific word choice of the addressee. (Contains 3 footnotes, 3 figures, and 1 table.)