A rise in anxiety after lying increases the temperature of the tip of your nose, say scientists.
If you're worried that your fib will be uncovered, the researchers also suggest a way of cooling the nose down - making "a great mental effort."
The scientists, from the University of Granada, discovered the phenomenon by using thermal imaging cameras to monitor volunteers, the Daily Mail reported.
They have called it "The Pinocchio Effect," in honour of the 19th century Italian tale of the wooden puppet whose nose grew when he failed to tell the truth.
In their doctoral thesis, Emilio Gomez Milan and Elvira Salazar Lopez suggest that the temperature of the nose increases or decreases according to mood, as does the orbital muscle area in the inner corner of the eyes.
The scientists also claim thermal imaging can detect sexual desire and arousal in men and women, indicated by an increase in temperature in the chest and genital areas.
And the technique also allowed the researchers to produce thermal footprints - body patterns with specific temperature changes - for aerobic exercise and distinct types of dance, such as ballet.
"When someone dances Flamenco, the temperature in their buttocks lowers and it rises in their forearms," the researchers said.
"This is the thermal footprint for Flamenco, although each type of dance has its own," they said.
The pair reached their conclusions after discovering that when the volunteers lied about their feelings, the brain's insular cortex was altered.
"The insular cortex is involved in the detection and regulation of body temperature, so there is a large negative correlation between the activity of this structure and the magnitude of the temperature change," they said.
"The more activity in the insular cortex (the higher the visceral feeling), lower heat exchange occurs, and vice versa," they added.