What's ONE thing can save your life and can kill you? Example: WATER.

Food

Electricity!  They can restart your heart with a ZAP from the paddles, or stop it with a ZAAAPPPP from a downed power line.  ⚡️

mcdonalds mcdoubles

Heat - it can stop you from getting hypothermia etc, but you can also die from heat stroke and other conditions

Good doctors can save you life.

Bad doctors might kill you.

A Marine.

things we take for granted that we use everyday. Creates pollution thus causes cancer

Airbags.

Man/ human beings

Guns can kill you or kill game to eat.

Air.

A dog. There are search and recue, therapy, service, police k-9, and bomb sniffing dogs. There are also dogs that not only been rescued, but rescued those that took them in. On the flip side there have also been those few who have attacked and killed people.

According to my doctor, Salt.  One lousy heart attack and they just won't let it go. 

A relationship.