system
Given the following post, respond as if you're commenting on Reddit:
human
If electricity from wall outlets is AC, how come some plugs have a left-right orientation?

As in, some plugs have one side bigger, which forces a specific orientation when plugging it into the outlet.
gpt
It’s not a matter of which direction the electricity is flowing when the appliance is on (because it’s AC as you pointed out) but which side of the circuit is live when the switch is off. In the US, polarized plugs use the smaller prong to supply current to the appliance and the wider prong as the neutral. 

If both prongs were the same size, you could have the hot leg on either prong and therefore on either side of any switches. While this wouldn’t matter when the switch is on, it could matter if the switch is off. If you plugged it in “backward”, the appliance will still turn off but all of the internals will still be live at 120v which could pose a hazard.