Why do snakes have forked tongues and why do they flick them? Is it because snakes are naturally deceitful, ready to whisper lies into passing ears? Actually, it's to aid a snake's vomeronasal system, or sensory system in snakes that is akin to smell. Snakes have forked tongues so that they can smell in stereo.

What exactly does this mean? What does it mean to sense anything in stereo?

If you listen to music holding only one side of your headphones to your ear, the whole orchestra sounds like it's crushed into one, tiny speaker. If you put your headphones on correctly, using both ears, the music becomes more three-dimensional. Likewise, it's easier to judge the depth of a scene with both eyes open than with only one. The key here, with both ears and eyes, is that your brain combines two different perspectives to make a more detailed, three-dimensional whole. Because you have two separate eyes and two separate ears, you can see and hear in stereo.

When a snake's tongue flicks out, the two tines of the fork spread as wide as they can. The tines flick back into the snake's mouth, and whatever chemicals each tine encountered in the environment are delivered to the snake's two, separate vomeronasal organs on the roof of its mouth. This gives the snake a directional perspective on the chemical traces in its environment, a kind of stereo smell. Like your two ears help you identify which direction a sound comes from, the two tines of a snake's tongue tell the snake whether its prey ran left or right.