How do fish know the way to streams they were born in to spawn? How do birds navigate for thousands of miles during migrations? Magnets! The Earth has a magnetic field that some animals can sense. Up until very recently, it was uncertain how animals sensed the magnetic field of the Earth. Now, scientists believe that animals have very special cells that are able to sense magnetic fields. These cells contain tiny magnetite particles that respond to a magnetic field. When the cells move (or rather, the particles in the cells move), a nerve impulse is sent to the brain. Animals interpret the signal in a way similar to their sense of touch.