Not all nouns that are identical in form to the present participle are gerunds. The formal distinction is that a gerund is a "verbal" noun – a noun derived from a verb that retains verb characteristics, that functions "simultaneously" as a noun and a verb, while other nouns in the form of the present participle (ending in "-ing") are "de"verbal nouns, which function as common nouns, not as verbs at all. Compare: