Tinker is an experimental interactive programming system which integrates program testing with program design. New procedures are created by working out the steps of the procedure in concrete situations. Tinker displays the results of each step as it is performed, and constructs a procedure for the general case from sample calculations. The user communicates with Tinker mostly by selecting operations from menus on an interactive graphic display rather than by typing commands. This paper presents a demonstration of our current implementation of Tinker.