> Ruby permits adding methods to individual objects; in Smalltalk, all methods reside in classes.
All Ruby methods reside in classes, too, "adding a method to an object" is really adding it to the object's singleton class (metaclass).
The difference between Ruby and Smalltalk is that in Smalltalk, each explicit class has an implicit metaclass (a class in which the class's instance methods are defined), whereas for Ruby each object (including, but not limited to, class objects -- and notably including other implicit metaclasses) has an implicit metaclass (often called the eigenclass).
And note that Ruby eigenclasses are in effect perfectly normal classes, with the exceptions that: 1) they dynamically get created the first time you define a method on it, 2) they are then "injected" into the inheritance chain, yet hidden from you when you try to follow the inheritance chain.
In effect, from an implementation point of view (at least for MRI), they're just normal classes except for when you define them, and a handful of insignificant cases where you have to check a flag to determine whether or not to consider them.
All Ruby methods reside in classes, too, "adding a method to an object" is really adding it to the object's singleton class (metaclass).
The difference between Ruby and Smalltalk is that in Smalltalk, each explicit class has an implicit metaclass (a class in which the class's instance methods are defined), whereas for Ruby each object (including, but not limited to, class objects -- and notably including other implicit metaclasses) has an implicit metaclass (often called the eigenclass).