Thank you! I used to use footnotes too, but I didn't like how they took you out of the flow of the text. Expounder aims to specifically let users stay in the flow of reading, which is why one of the core instructions is that the text should work in context, as if it were never hidden.
It's good to see experiments along these lines. I really like Wikipedia's recent-ish rich tooltips on link mouseover, and the HTML <summary>/<details> elements deserve to be more widely known.
From the demo it look as if Expounder is one-way - once you've expanded something, you can't collapse it again. Is that correct?
I miss footnotes on the printed page because, in addition to references (where they're probably better as endnotes to be honest) I find they're great to use for parentheticals that bulletproof a point, add some background that's not essential to a point being made, etc. But these latter uses work significantly less well in a blog post or ebook.
