Some animations I demoed in 1986 illustrated that you can't tell if an enlarging hyperbola is scaling anisotropically. One of them was actually stretching horizontally and shrinking vertically at a lesser rate, which only became clear as the background faded up some crazy chewing gum wallpaper. Thus an apparently fixed hyperbola might be stretching one way and shrinking the other at complementary rates. This is shown by Neil's vastly subtler chewing gum: gosper.org/hyperbolicV2.gif . Staring at this for a few periods and then suddenly freezing it produces a strong and amusing variation on the moving bar illusion. The problem: how to freeze an mgif without moving your eyes? https://addons.mozilla.org/en-US/firefox/addon/toggle-animated-gifs/ enables Firefox to alternately play and pause mgifs with the keystroke Control-m. --Bill Gosper