04-lists.md: improve introduction

Pull Request: #567

Author: nathaliagg

_episodes/04-lists.md

@@ -20,9 +20,17 @@ list[2:9]), in the same way as strings and arrays."
 - "Strings are immutable (i.e., the characters in them cannot be changed)."
 ---
 
-Similar to a string that can contain many characters, a list is a container that can store many
-values. Unlike NumPy arrays, lists are built into the language (so we don't have to load a library
-to use them).
+In the previous episode, we analyzed a single file with inflammation data. Our goal, however, is to
+process all inflammation data we've got, which means that we still have eleven more files to go!
+
+The natural first step is to collect the names of all the files that we have to process. In Python,
+a list is a way to store multiple values together. In this episode, we will learn how to store
+multiple values in a list as well as how to work with lists.
+
+## Python lists
+
+Unlike NumPy arrays, lists are built into the language so we don't have to load a library
+to use them.
 We create a list by putting values inside square brackets and separating the values with commas:
 
 ~~~
@@ -57,22 +65,6 @@ Yes, we can use negative numbers as indices in Python. When we do so, the index
 last element in the list, `-2` the second to last, and so on.
 Because of this, `odds[3]` and `odds[-1]` point to the same element here.
 
-If we loop over a list, the loop variable is assigned to its elements one at a time:
-
-~~~
-for number in odds:
-    print(number)
-~~~
-{: .language-python}
-
-~~~
-1
-3
-5
-7
-~~~
-{: .output}
-
 There is one important difference between lists and strings:
 we can change the values in a list,
 but we cannot change individual characters in a string.
@@ -308,33 +300,6 @@ odds: [1, 3, 5, 7]
 ~~~
 {: .output}
 
-> ## Turn a String Into a List
->
-> Use a for-loop to convert the string "hello" into a list of letters:
->
-> ~~~
-> ['h', 'e', 'l', 'l', 'o']
-> ~~~
-> {: .language-python}
->
-> Hint: You can create an empty list like this:
->
-> ~~~
-> my_list = []
-> ~~~
-> {: .language-python}
->
-> > ## Solution
-> > ~~~
-> > my_list = []
-> > for char in 'hello':
-> >     my_list.append(char)
-> > print(my_list)
-> > ~~~
-> > {: .language-python}
-> {: .solution}
-{: .challenge}
-
 Subsets of lists and strings can be accessed by specifying ranges of values in brackets,
 similar to how we accessed ranges of positions in a NumPy array.
 This is commonly referred to as "slicing" the list/string.

_episodes/05-loop.md

@@ -98,7 +98,7 @@ IndexError: list index out of range
 ~~~
 {: .error}
 
-Here's a better approach:
+Here's a better approach: a [for loop]({{ page.root }}/reference/#for-loop)
 
 ~~~
 odds = [1, 3, 5, 7]

_extras/extra_exercises.md

@@ -49,6 +49,33 @@ or not (yet) added to the main lesson.
 > {: .solution}
 {: .challenge}
 
+> ## Turn a String into a List
+>
+> Use a for-loop to convert the string "hello" into a list of letters:
+>
+> ~~~
+> ["h", "e", "l", "l", "o"]
+> ~~~
+> {: .language-python}
+>
+> Hint: You can create an empty list like this:
+>
+> ~~~
+> my_list = []
+> ~~~
+> {: .language-python}
+>
+> > ## Solution
+> > ~~~
+> > my_list = []
+> > for char in "hello":
+> > 	my_list.append(char)
+> > print(my_list)
+> > ~~~
+> > {: .language-python}
+> {: .solution}
+{: .challenge}
+
 > ## Fixing and Testing
 > From: "Defensive Programming"
 >