system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do Geodes form and why are they hollow?
gpt
When rocks get really, really, *really* hot, they turn into a liquid... but it isn't always a runny liquid like water. Most of the time, in fact, it's much thicker; almost like pudding. (You wouldn't want to eat it, though!)

Now, deep beneath the Earth's surface, there's *a lot* of liquified rock sloshing around, and sometimes, it gets forced up to where we can see it. It quickly cools and solidifies at that point, but before it does, it often traps bubbles of air within it.

If these pockets of air are in the right place, are the right shape, and are left alone for long enough, rain and other weather will very slowly fill them with water. That water will drain away just as gradually, and it will leave behind tiny deposits of minerals. Some of these minerals are called "silicates" (the same stuff that sand is made out of), and as they build up, they form into crystals. Calcium carbonate is another mineral that can seep in and grow into crystals, but the popular purple geodes – what we call "amethyst geodes" – are silicates. 

That purple color is actually irradiated iron. (Don't worry, though: Iron is the most-stable element that we know of, so even the irradiated sort is usually less radioactive than even carbon.) Just like the silicates, the iron is brought into the air pockets by water... and after many, many years, the substances bind together to create the purple crystals that we all know and love.

That takes a very long time, of course, and *usually*, there's some empty space left over in what used to be the pocket of air. That's why geodes are hollow!