Before starting the experiment, what does Dr. Heidegger remind each of his guests?
Dr. Heidegger's experiment is conducted with four willing participants: the Widow Wycherly, Colonel Killigrew, Mr. Medbourne, and Mr. Gascoigne. All are of an advanced age and eager to try a substance he identifies as fluid from the Fountain of Youth. Because all four have made poor choices in their past and suffered negative consequences, Dr. Heidegger counsels them to use the wisdom gained from their life experience and "become patterns of virtue and wisdom to all the young people of the age!" Because Dr. Heidegger believes he understands human nature and expects that the four participants in his experiment will make the same mistakes, he offers this advice.
The four find his advice "ridiculous" and unnecessary; the implication is that anyone given the chance to relive their past would naturally sidestep their youthful mistakes and missteps to avoid trouble and regret.