system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does defogging the windshield work?

I feel dumb for this, but I never know what temperature to put the air to. Also why doesn’t it help when I turn the AC off but still have the fan blowing air from outside at the window? Shouldn’t that equalize it?? 

Edit: RIP my inbox. Thank you for so many answers. Please do not share more Mark Rober links there are like 5 in the comments already! If you want more of an in-depth answer there are several recommendations down there to look him up so do it!
gpt
Think of air as a sponge. However the temperature of the air determines the size of that sponge and how much liquid it can hold. The colder it is, the less water the sponge can hold. The air always has some amount of water in it, this is air humidity. When the air touches the cold windshield, the "sponge" shrinks and suddenly can't hold all the water it was holding. This extra water is dropped off and left on the windshield. This is fog.  
  
In order to defog the windshield you need to remove the moisture. The best way to do this is to use a big dry sponge, (warm, dry air). Generally the humidity inside the car is higher than outside so ideally, you want to use warm air that is coming from outside with the AC on.  
  
Part of what the Air Conditioning does is reducing the humidity. So when you turn the AC off and just blow outside at the windshield, this will eventually work, but is left efficient. An important note, AC doesn't always refer to cold air. The AC can be on while hot air is coming out too.