The formal definition of the limit is probably the most abstract (and hence most difficult) concept we will discuss in Calculus I. I found an interactive diagram at [url]http://www.scottsarra.org/applets/calculus/EpsilonDelta.html[/url] that does this well. It is part of a larger collection: [url]http://www.scottsarra.org/applets/applets.html[/url] . I also produced one myself with GeoGebra that you find below.