Doesn't work for me on Firefox.

I'm guessing it works on Chrome, but it also would sort of surprise me that a maker website wouldn't test against Firefox. Maybe I'm giving them too much credit.

It took me a minute to figure out, you have to drag the circle up and down to select the number. I had expected to drag the whole bar - oops!

Edit: Also on FF. (85.0 on Linux)

I finally figured it out!

Literally the only thing I didn't try moving was that circle blob.

It definitely could be made more discoverable, maybe with a bigger "handle" around it to show draggability. But I really love the idea of a combined educational tool and spam deterrent.

Strange. I'm on Firefox and it works for me.