Answer:
At the end of the story "Flowers for Algernon" by Daniel Keyes, Charlie Gordon's intelligence regresses, and he returns to a state of intellectual disability. His decline is a result of the temporary nature of the experiment that had initially boosted his intelligence. As his mental abilities diminish, Charlie becomes aware of his regression and is left with a poignant sense of loss and the realization that he will never again experience the level of intelligence he had briefly attained. The story ends on a somber note as Charlie's mental faculties deteriorate, and he loses the gains he had made, ultimately returning to the cognitive state he had at the beginning of the story.