Salmon’s built-in compass*
How do salmon know how to get back from the ocean to the exact place they spawned when it’s time to make sweet, sweet love again? Built-in magnets, of course. Well, “microscopic crystals of magnetite in their tissue” to be specific.
“[T]hese salmon know where they are, where they’re supposed to be, how to get there and how to make corrections if needed. While they’re in fresh water, they’re imprinting upon the chemical nature of the water. When they hit salt water, they switch over to geomagnetic cues and lock in that latitude and longitude, knowing they need to come back to those coordinates.”