Mostly because it looks like Java and on the first glance also shares Java's mostly static type system (which is not strictly true). The gist of it is: you can

Mostly because it looks like Java and on the first glance also shares Java's mostly static type system (which is not strictly true). The gist of it is: you can send arbitrary messages in Smalltalk (and ObjC) and the object itself deals with them somehow (with most of that being implemented by runtime anyway, but that is different issue). In C# you call methods defined by interface. Also, you cannot modify arbitrary parts of standard library and runtime, but that is probably for the better (doing that is pretty common in ST world).