When using an oral temperature device, what must be done with the mouth?

The correct practice when taking an oral temperature is to keep the mouth closed. This helps ensure that the measurement is accurate, as an open mouth can allow for the loss of heat and disrupt the reading. When the mouth is closed around the thermometer, it stabilizes the temperature within the oral cavity, allowing the device to register the most accurate temperature of the body's core.

In contrast, options such as opening the mouth or leaving it partially open would lead to potential errors in temperature measurement due to airflow or exposure to cooler ambient air. Similarly, leaving the mouth uncovered would also not support an accurate reading, as it does not provide an enclosed environment for the temperature gauge. Thus, maintaining a closed mouth is crucial for obtaining reliable oral temperature readings.