"Dr. Heidegger's Experiment," a short story written by Nathaniel Hawthorne, was first published anonymously in 1837 as "The Fountain of Youth." Later that year, the story was published again, with its present title and under Hawthorne's own name, in a collection of stories entitled Twice Told Tales—"Twice Told" meaning that each of the stories had been published previously.

"Dr. Heidegger's Experiment" is told in a conversational narrative style. The narrator refers to himself in the first person ("I") and to the reader in the second person ("you), which implies a somewhat informal or even confidential relationship between the narrative and reader.

The story involves Dr. Heidegger, a "very singular" medical doctor, and four very old friends whom Dr. Heidegger has invited to his home and into his "very curious" study to participate in an experiment with water which Dr. Heidegger claims is from the "real Fountain of Youth."

Dr. Heidegger's four guests agree to drink the water, which restores each of them to "the happy prime of youth."

They felt like new-created beings in a new-created universe.

"We are young! We are young!" they cried exultingly.

In time, the youth-imparting effect of the "Water of Youth" wears off.

They stood still and shivered; for it seemed as if gray Time were calling them back from their sunny youth, far down into the chill and darksome vale of years....

A strange chillness, whether of the body or spirit they could not tell, was creeping gradually over them all....

"Are we grown old again, so soon?" cried they, dolefully.

Having observed the effects of the "Water of Youth" on his friends, Dr. Heidegger resolves not to partake of its magical properties.

But the doctor's four friends had taught no such lesson to themselves. They resolved forthwith to make a pilgrimage to Florida, and quaff at morning, noon, and night, from the Fountain of Youth.

Elements of the story might strain credibility, which the personable narrator confides to the reader early in the story, in paragraph 6:

Now Dr. Heidegger was a very strange old gentleman, whose eccentricity had become the nucleus for a thousand fantastic stories. Some of these fables, to my shame be it spoken, might possibly be traced back to my own veracious self; and if any passages of the present tale should startle the reader's faith, I must be content to bear the stigma of a fiction monger.

It might well be, implies the narrator, that he made up the whole story, but he's content to leave it up to the reader to decide which "passages" of his story are true and which are entirely fictional.