system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some US electrical plugs have a "ground" and many do not?

I don't know much about how plugs or electricity works, obviously, but I was taught that one side is the "positive", one side is the "negative", and the bottom (seemingly quite optional) is the "ground". It's odd to me that so few plugs use the "ground", so it made me curious why it exists, and why it's optional. Are there any safety benefits to having a "ground", or safety concerns with not having one? Thank you!
gpt
Positive and negative doesn't really apply here, because they switch back and forth 60 times per second. You have hot and neutral. Hot is connected to one of the phases, neutral is the same potential as ground, but only connected to ground in the main breaker box. stuff that uses a lot of power will often have two hot wires with 240v between them, and the neutral wire is halfway in between.  If you have an appliance with a metal case, you connect the ground wire to the case, so that in the event of a failure somewhere, the outside of the device shorts that back to ground and trips the breaker, instead of sitting there at mains voltage waiting for someone to touch it.

Two prong plugs are used for double insulated appliances, usually items with plastic shells that won't conduct electricity if there's a failure, but it can also apply to metal items so long as a single fault won't cause an electrocution hazard.