Add a more elaborate description to the side effects section

README.md

@@ -549,17 +549,24 @@ If you can do this, you will be happier than the vast majority of other programm
 **Bad:**
 
 ```python
-# Global variable referenced by following function.
-# If another function used this name, now it'd be an array and could break.
+# This is a module-level name.
+# It's good practice to define these as immutable values, such as a string.
+# However...
 name = 'Ryan McDermott'
 
 def split_into_first_and_last_name() -> None:
+    # The use of the global keyword here is changing the meaning of the
+    # the following line. This function is now mutating the module-level
+    # state and introducing a side-effect!
     global name
     name = name.split()
 
 split_into_first_and_last_name()
 
 print(name)  # ['Ryan', 'McDermott']
+
+# OK. It worked the first time, but what will happen if we call the
+# function again?
 ```
 
 **Good:**
@@ -586,6 +593,7 @@ class Person:
     def name_as_first_and_last(self) -> list:
         return self.name.split() 
 
+# The reason why we create instances of classes is to manage state!
 person = Person('Ryan McDermott')
 print(person.name)  # 'Ryan McDermott'
 print(person.name_as_first_and_last)  # ['Ryan', 'McDermott']