Living languages are always changing; the most conspicuous way they change is in their sounds, and this change is generally regular — which is why it’s possible to imitate another dialect even if you’ve never heard the particular words spoken in that dialect.
Geoff’s Sound Change Applier (link updated 2018) is a swell toy. You feed it a word in the parent language, and a list of rules for changes between that and a descendant language, and SCA predicts the descendant form of that word. For a sample, Geoff includes a (crude) list of rules to transform Latin words into Castilian words. This way you can quickly test the accuracy of a given reconstruction of such changes.
Those who play at inventing languages (a sport whose most famous player was Tolkien) can use SCA to generate a whole family of languages. What fun.