system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do nuclear explosions take the shape of mushroom clouds? Do all nukes explode in that shape?

As I've yet to see a nuclear explosion in real life, I only have movies, TV, and archive video go off of. 

Obviously, not ALL nukes explode this way, right? Why are they constantly depicted as being that shape in live action movies and cartoons?

Edit: Thanks for all the answers, you guys! Also, this thread is a goldmine if you have the 'Cloud to Butt' extension in Chrome.
gpt
When a nuke goes off, it creates a giant shockwave that very quickly moves outward in a sphere. If it's close enough to the ground, then some of it bounces off of the ground and you get a sort of double shockwave traveling along the Earth's surface. This kicks up lots of dust, especially if the fireball itself hits the ground and vaporizes even more stuff. 

Anyways, this shockwave is really powerful, and one of the things it does is that it pushes a lot of the air away from the explosion. This happens really quickly, and it leaves behind an area of much lower pressure. 

So once the shockwave moves away, then air starts rushing back from all directions, in towards the source of the explosion. When it all meets at the center, it collides and has nowhere to go except up. And so that forces all of the dust up to make the mushroom stem. As the air/dust mixture gets higher up and past where all the air was rushing back in, it starts to spread out and make the mushroom "cap".