Final answer:
The oral thermometer typically features a red dot, signifying the presence of red dye in the alcohol used within the thermometer. This color aids in reading temperature changes as the red-colored liquid expands and moves up the narrow tube in response to heat.
Step-by-step explanation:
The oral thermometer is marked with a red dot. This is because, in common types of thermometers, the alcohol that is used to measure temperature contains a red dye which expands more rapidly than the glass encasing it. When the temperature rises, the expansion of the alcohol forces it into a narrow tube, thereby increasing the length of the column and indicating the temperature change.
The use of red dye is not simply a random choice; red often signifies warmth or heat, which makes it a practical and intuitive color to represent an increase in temperature. This is similar to how water faucets are color-coded, with a red dot typically signifying hot water.