A 3-year-old boy presents with acute watery diarrhea for 2 days and clinical signs of moderate dehydration. His mother reports that he has been passing 8–10 stools per day, with no blood or mucus. On examination, he has a normal mental status, drinks eagerly when offered fluids, and has a capillary refill time of 2 seconds. Which single clinical feature best distinguishes moderate dehydration from severe dehydration in this child?