There are behaviours which are innate and may not require any conscious thought. Escape reflexes, taxes (sg. taxis) and kineses (sg. kinesis) are types of innate behaviours that help organisms avoid danger, find food and respond to environmental stimuli quickly enough to be effective.
Reflex – think of sensory neuron –> interneuron –> motor neuron
The knee-jerk reflex is too famous so it shall be ignored on this occasion (also, what is the point of it anyway? I never recall it saving me from anything, do you?!). Instead we turn to the iris reflex responsible for controlling the amount of light entering your eyes.
(Fun experiment that’s also an opportunity for justified procrastination: go in the bathroom and keep covering an eye and uncovering quickly while staring in the mirror to see how quickly the pupil appears to…