This is a side project, and the goal is not to teach how to code. I agree with several comments here, it's sometime confusing, it's short and minimalistic. The real goal is far less ambitious. I just wanted people to catch a glimpse of what is coding. It's like stepping into un unknown world for a few minutes. And it's not aimed at everybody. Some people are driven away by written explanations, or can feel patronized very quick, and I wanted to address that. Here, the first questions make you feel confident, then with tries and errors, by yourself (or apparently so) you understand a bit how this works.
The story behind this is : I was never able to explain how code works to my wife. She is math-o-phobic and she never gave me the chance to explain... just the fact of explaining something like this scares her away. But one day, sitting next to her on the train, I opened my JavaScript console and began typing 3 + 4, then without a word I pointed the keyboard and screen to her... 20 minutes later we were doing functions and variables, without even a word. This is how this project was born.
Sorry for those who where mislead by the title of this HN post. We have other tools currently in development to address code teaching (one more focused on code reading, one more focused on creativity, one more focus on JavaScript ...), and only when 3 or 4 of these will be published will we be able to teach properly to a larger audience.
Thanks for all the feedback today, I really appreciate it.