system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is it that determines whether a third prong is needed on power plugs for tools and appliances?

First of all, I’m in the US, if that matters. 

On power plugs that stick into wall outlets, there are always two prongs that allow for electricity flow. There is sometimes a third prong, but not always. 

What are the criteria that determine whether a third prong is necessary? My theory as a layperson is that it’s for electrical grounding and it must have to do with the amount of power being used, the nature of the tool/appliance, or maybe both. But I can’t figure out what exactly that factor is. Similar things, like two vacuum cleaners of approximately the same size, would presumably require about the same amount of power, but sometimes one will have a third prong and another won’t. I guess one of them might be just a bit stronger than the other and require enough additional power that it crosses some threshold beyond which the third prong is suddenly necessary. But it seems kind of arbitrary for someone like me who doesn’t know the intricacies of electrical engineering.
gpt
Designers of electrical gadgets have a choice, they must provide internal shock protection.  One choice is to ground all the conductive parts the operator can touch, and doing that requires a ground wire, and thus a third prong on the plug.  The other choice is double insulation, where every part the operator can touch is non-conductive.  Some things just can't do their job and be made of plastic, so some gadgets always have a third prong.  There are other odd cases, like the blow-dryer with the GFCI in the plug, but those are for specialized situations.