When is a Gerund NOT a Gerund?

Hands up all those who know what a gerund is!

a. The book was very interesting.
b. Reading is my favourite activity.
c. At this moment I'm reading a great book.

Which '-ing' form here is a gerund? Normally my students say the -ing form of a verb, no matter how it's used, is a gerund, perhaps in Latin languages this is the case (verb forms ending in -indo, -ando or -endo in Portuguese) This is not necessarily so in English, the -ing form of the verb is called a present participle. A present participle can take on the role of an adjective (example a.), a noun (b.) or a verb in a continuous tense (c.). When it is used as a noun, the present participle is a gerund, so from our three examples, b. Reading is my favourite activity; reading here is a gerund, like you would say, Pizzas are my favourite food or I like pizzas and reading.