In this article, Devin Rader explains the concept of using HTTP cookies to track SMS conversations in Twilio applications. He begins by explaining how cookies work in a typical web browser/web server scenario and then shows how Twilio uses cookies differently. The author then builds a simple conversation tracking app using Flask and the Twilio Python helper library, demonstrating how to use persistent cookies to store state between messages. Additionally, he discusses using session data instead of cookies for larger amounts of transient data. Finally, Rader addresses scenarios where an application initiates a conversation by sending an initial SMS message using Twilio's REST API, highlighting why traditional cookie-based approaches won't work in these cases. Throughout the article, the author emphasizes that Twilio is a well-behaved HTTP client, and techniques used for cookies on websites can be applied to Twilio applications with minimal modifications.