When animals gets scared, their first reaction is typically to stop dead in their tracks, as though the light's just turned red in a game of Red Light-Green Light. In many cases, this helps the animals go unnoticed by nearby predators that hunt by sensing movement.

When animals freeze, their eyelids open wide, the pupils of their eyes dilate and certain muscles in their eyes relax, helping them to take in more detail about their surroundings. Animals may make use of the time they gain while staying still to look for possible getaway routes or to prepare for an unavoidable fight.