The snake’s tongue has a fork on the end of it, because it captures little pieces of smell — odor particles — that are floating in the air. The fork in the tongue that holds this smelly air is brought back into the snake’s mouth and pressed against the roof of the mouth.
The snake has an organ called the Jacobson’s organ inside its head. When the snake’s tongue goes back inside its mouth, it is put into two pits in the roof of its mouth. Those two pits are the entrance to the Jacobson’s organ. The two pits in the roof of the mouth is why snakes have to have that forked tongue.
The air particles that are pressed into the two pits in the roof of the mouth have information that is sent into the Jacobson’s organ. This special organ reads the information about the air’s scent and then sends that information to the snake’s brain.
Image courtesy of Fred the Oyster, Wikimedia Commons
That is how a snake’s tongue is used to smell!
Snakes also have nostrils on top of their heads to smell and their noses work with the scents collected by the Jacobson’s organ. This makes a snake’s sense of smell superpowered compared to you or me.
Cats also have a Jacobson’s organ, and they often use it to read messages in pheromones — scent chemicals — from other cats, especially those found in urine. Those scent chemicals are often a way for cats to mark what is their territory or to see if a female is ready to mate.
You can tell a cat is using this sense of smell instead of its nose when it curls its upper lip and keeps its mouth partially open while it flicks its tongue out.
The Jacobson’s organ is also found in other lizards, some salamanders and other mammals.