How do you explain PTSD "triggers" to someone?

"A PTSD trigger is any sensory event(sound, smell, sight, touch) that elicits an immediate, and sometimes unconscious somatic stress(survival) response in a person with PTSD. The trigger is perceived as an immediate threat to survival, and that triggers a hard-wired response in the brain to get the rest of the body to confront the threat "fight", to get the hell away from it "flight", or to avoid it, possum-style "freeze". "The fight-flight-freeze stress response is incredibly fast. It activates the body IMMEDIATELY. The brain's messages floods the body with several stress hormones(including epinephrine, norepinephrine, cortisol) which increase heart rate, breathing rate, blood pressure, blood sugar levels, blood flow to skeletal muscle, and cause "tunnel vision" which helps to hyper focus on the threat. They also decrease nonpriority survival processes such as digestion and libido. The stress response is biologically expensive. It is a last-resort effort to maintain survival. It beats the body up. It is not supposed to be something that occurs everyday. But it does in a person with PTSD. Now, imagine how utterly exhausting it must feel to experience this on a near-daily basis. Your heart is racing, your muscles are tense, you're jumpy and short-tempered, ready to attack or hide. You can't concentrate on anything other than the threat. Or you zone out and are unable to feel your body or where you are spatially. A "normal, everyday" sound, smell, sight or touch that the survival center of your brain associated with a lethal threat during the traumatic event now evokes the overwhelming, whole-body stress response. "It's difficult to describe such a perception and response, especially since it's not within the scale of "normal". It's like trying to describe the color red to a person who is blind." Answered on Quora by Niamh XXXX