Framed within the field of Cognitive Linguistics, Galera concentrates on idioms, here defined as pervasive, non-literal expressions that bear a certain degree of fixation and whose meanings need to be deciphered in terms of cognitive operations. Hence, following authors like Ruiz de Mendoza and Pérez (2003), Ruiz de Mendoza and Peña (2005) or Ruiz de Mendoza (2010), the aim of this paper is to corroborate that idioms, and more particularly simile-based idiomatic expressions (e.g. I” felt like a fish out of water”), should be analyzed in terms of cognitive operations like metaphor and metonymy, as well as on the basis of a combination of both mechanisms. After providing the reader with an exhaustive overview of the literature devoted to the issue at hand, the author presents an in-depth analysis of examples such as “He is like castanets”, “John is a pig”, “John sweats like a pig”, etc., discussing the type of cognitive operation(s) underlying them. These may involve a metonymic expansion of a single-source metaphorical complex, a metonymic expansion of the source and target metaphoric domains, a metonymic expansion of the source domain, a metonymic expansion of the metaphorical target domain, to name a few cases.