ELI5 Stress - What does it do to our bodies?

What causes stress? What does it do to our bodies? What’s the difference between 'fight or flight' and 'rest and digest'? Does stress cause grey hair?   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: 4dddyq, windystew & penakazato   To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]