The R language is peculiar in many ways, and its approach to object-oriented (OO) programming is just one of them. Indeed, base R supports not one, but three different OO systems: S3, S4 and RC classes. And yet, probably none of them would qualify as a fully-fledged OO system before the astonished gaze of an expert in languages such as Python, C++ or Java. In this tutorial, we will review the S3 system, the simplest yet most elegant of them.