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100-level-AsyncAPI-Intro/step3.md

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+When Chan goes back to his planet, he wants to communitcate with Eva. For communicating he needs to learn how to write AsyncAPI documentation and check if he can send a message to Eve.
+
+### Writing an AsyncAPI Document from Scratch:
+
+#### Setting Up the Basics:
+
+Chan begins by setting up his space message plan. He writes:
+
+```
+asyncapi: 3.0.0
+info:
+  title: Sending Signal to Eve
+  version: 0.1.0
+```
+
+This is like Chan saying, "I'm using the 3.0.0 space message rules, and I'm naming my plan `Sending Signal to Eve.`"
+
+#### Creating Channels and Messages
+
+Chan then decides on the path his message will take to Eve. He sets up a channel, like a route in space, and names it `userSignedUp` with an address `Earth/letter`. The document looks like:
+
+```
+channels:
+  userSignedUp:
+    address: Earth/letter
+    messages:
+      lettertoEarth:
+        description: Communicating with Eve.
+        payload:
+          type: object
+          additionalProperties: false
+          properties:
+            fullName:
+              type: string
+            email:
+              type: string
+              format: email
+            age:
+              type: integer
+              minimum: 18
+```
+
+This part is like Chan detailing the contents of his message – like who it's from (full name), how to contact back (email), and making sure the receiver is old enough (age).
+
+#### Setting Up Operations
+
+Finally, Chan adds instructions for how the message should be sent. He writes:
+
+```
+operations: 
+  userSignedUp:
+    action: send
+    channel: 
+      $ref: '#/channels/lettertoEarth'
+```
+
+This tells his spaceship that it should `send` messages through the `userSignedUp` channel.
+
+### Using AsyncAPI Tools for Validating the Document
+
+After writing his space message plan, Chan uses a tool called AsyncAPI Studio to make sure everything is set up correctly. It's like a helper that checks his work to make sure there are no mistakes.
+
+To use AsyncAPI Studio, Chan would first install it using the command `npm install -g @asyncapi/studio`. Then, he would open the AsyncAPI document in the studio using the command `asyncapi studio my-document.yaml`. The studio would then check the document for errors and provide suggestions for improvements.
+
+### Hands-on Exercise
+
+Chan wants to practice more. He thinks about adding a new property for a username and creating a new channel for messages to Mars. He updates his document with these new details and then uses AsyncAPI Studio to validate his changes.
+
+#### Expected Outcome
+
+After following these steps, Chan should be able to see his updated document in the AsyncAPI Studio. The studio should validate the document and show any errors or warnings.