articles/oop.md: update article

articles/oop.md

@@ -1,12 +1,13 @@
 # Object Oriented Programming
-Also known by its acronym OOP (**POO** in Spanish for *Programación Orientada a Objectos*) is a programming paradigm.
+Also known by its acronym OOP (**POO** in Spanish for *Programación Orientada a Objetos*) is a programming paradigm.
 
 OOP is split between two "schools" of design: Simula-style OOP and [Smalltalk](smalltalk.md)-style OOP.
 
 TODO
 
 ## Simula-style OOP
-Most popular languages that have OOP have it in the Simula form. Some of these languages are:
+Most popular languages that have OOP have it in the Simula form, so when the term OOP is used, one always means this kind of OOP.
+Some of these languages are:
 - [C++](cpp.md)
 - [Java](java.md)
 - [JavaScript](javascript.md)
@@ -15,7 +16,20 @@ Most popular languages that have OOP have it in the Simula form. Some of these l
 
 ## Smalltalk-style OOP (message passing)
 In Smalltalk-style OOP, objects communicate with each other by means of message passing. This
-allows for greater dynamism, something that on Simula-style OOP can be archived with virtual methods.
+allows for greater dynamism. Unlike in Simula-style OOP, objects can receive any message and its
+up to the object whether to reply or raise an error if it doesn't understand the message.
+
+For example, in Smalltalk if I have an object `x`, I can send messages to it and it will reply
+with an answer:
+```st
+| x |
+x := 39.
+"Are you `nil`?"
+x isNil.                "==> false"
+"Messages can carry a payload of course:"
+"Are you a number?"
+x isKindOf: Number.     "==> true"
+```
 
 Some languages based on or inspired by Smalltalk-style OOP are:
 - Objective-C: some [C](c.md)-Smalltalk chimera; similar to C++ but using message passing instead of method calling.