I learned a great trick from our Sketchpad developers that some of you may appreciate. If you have a sketch that you'd like your students to use on the iPad, you can place scroll limiters in your sketch so that they don't accidentally scroll off into infinity. Simply place two points in opposite corners of your sketch, where you want the scrolling to be stopped, and label them "Scroll Limiter". Be sure to have a space in the middle, and the words capitalized. This will also limit zooming out.