What do reptiles and mammals use their Jacobson's organ to detect?

The Jacobson's organ is located on the roof of the mouth in reptiles and mammals. It is also called the vomeronasal organ. This organ works by sensing the scent of chemicals such as pheromones.

Reptiles and mammals use their Jacobson's organ to sense pheromones. Elephants touch the tips of their trunks to this organ to enact their chemosensory perception of things. A lion uses it for sensing sex hormones, and will often open its mouth to sniff the pheromones it senses.

The Jacobson's organ also helps some animals perceive other chemical compounds besides just pheromones emitted between species. For instance, snakes find their prey by using it. A snake places its tongue on the two pits in the roof of its mouth after having its tongue in the air to allow it to properly sense the direction of its prey. The reason snakes have a forked tongue is so that the tongue can touch these pits. The deeper the fork in a snake's tongue, the more the snake uses its Jacobson's organ.