Final answer:
A rectal thermometer should be inserted gently about half an inch to one inch for infants and up to two inches for adults to measure temperature accurately and avoid injury.
Step-by-step explanation:
When using a rectal thermometer to measure body temperature and avoid injury, it should be inserted gently into the rectum only about half an inch to one inch (1.25 to 2.5 centimeters) for infants and up to two inches (5 centimeters) for adults. It's crucial to follow these guidelines to prevent any harm while ensuring an accurate reading. If discomfort or resistance is met, stop the insertion and ensure proper technique and lubrication are used.