First I have to say, THANK YOU for introducing me to this!
I absolutely love playing around with changing variables.
So much so that I've already made something.
I introduced the first of a couple of culminating projects yesterday. This one is creative art on Desmos using the functions we have studied. It's a good fit because we emphasize domain, range, and transformations all year. So as they create art, I require a particular window ... a frame for their art ... that forces transformations. And of course, to create art, students have to limit the domain of each equation to get the shape they want. Some students, like the young man in the email, really like the creative process and the responsiveness of Desmos!