diff --git a/episodes/02-storage.md b/episodes/02-storage.md
index 1823c46..34b0bd2 100644
--- a/episodes/02-storage.md
+++ b/episodes/02-storage.md
@@ -92,6 +92,10 @@ If this is your first project or you created it from the Google Cloud Console We
 
 If you created the project from the command line without specifying the billing account, you must link it to an existing billing account.
 
+:::::::::::::::::::::::::::::::::::::::::: spoiler
+
+### How to link a project to your billing account?
+
 First list the billing accounts
 
 ```bash
@@ -110,6 +114,8 @@ If not, link your project to this account with
 gcloud billing projects link <PROJECT_ID> --billing-account <ACCOUNT_ID>
 ```
 
+::::::::::::::::::::::::::::::::::::::::::::::::
+
 ### Choose your region
 
 In this tutorial, we will use the Google computing centre `europe-west4` located in Netherlands. You can use another one, but use it consistently.
@@ -170,7 +176,7 @@ In the context of the example of this tutorial, the resulting output files are a
 
 - Google Cloud Storage bucket can be used to store the output files.
 - The storage cost depends on the volume stored and for this type of processing is very small. 
-- The download of the output files for the bucket has a significant cost.
+- The download of big output files from the bucket can be costly.
 
 
 ::::::::::::::::::::::::::::::::::::::::::::::::
diff --git a/episodes/03-disk-image.md b/episodes/03-disk-image.md
index b7c666d..1cb9151 100644
--- a/episodes/03-disk-image.md
+++ b/episodes/03-disk-image.md
@@ -17,16 +17,19 @@ exercises: 5
 
 ::::::::::::::::::::::::::::::::::::::::::::::::
 
-## Work in progress
+## Why a disk image?
 
-Building the disk image does not work for a new GCP account for the moment.
+
+The container image for the MiniAOD processing is very big and it needs to be pulled to the nodes of the cluster. It can take 30 mins to pull it.
+Making it available to the nodes of the cluster through a pre-built secondary disk image speeds up the workflow start.
+
+A script is available but building the disk image does not work for a new GCP account for the moment.
 
 Investigating....
 
 
 ## Prerequisites
 
-
 ### GCP account and project
 
 Make sure that you are in the GCP account and project that you intend to use for this work. In your Linux terminal, type