diff --git a/README.md b/README.md
index 21d2b43..811a270 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,15 @@
 
 Made to talk with a friend without eavedroppers.
 
-## Usage
+## How does it work?
 The tool uses a numerical key to shift the message's characters.  
 Only alphabetical characters are shifted.
+
+Say the key is '1234' and the message is "Hello world".  
+We start by shifting "H" to the right once: "I".  
+Then we shift "e" to the left twice: "c".  
+We keep doing this until we get: "Icohp urnmb".  
+
+The alphabet and key are wrapped around.  
+So the actual key is `123412341234123412`...  
+