Why snake sticks out its tongue?

 The tongue of the snake captures the little pieces of smell that are floating in the air.

This tongue that holds the smelly air is brought back to the mouth of snake and pressed against the roof of the mouth.

When the snake's tongue go back to its mouth, it is put into 2 pits.

The air particles that are pressed in these 2 pits, sends the information to the brain of snake.

This is how a snake's tongue is used to smell.

Snake's like python have their nostrils on top of their heads.