system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The Boltzmann Brain Theory.
gpt
Take a puzzle with 1000 pieces, throw them on the ground. You'll probably generate a huge mess. Repeat this often. Sometimes two pieces that fit together will land next to each other. Repeat this really, really often. Once in a while, all 1000 pieces will land at their right spot.

Our brain is a bit like a puzzle - just with even more pieces (atoms). Space is never completely empty, there are a few atoms and radiation flying around, so space is constantly "throwing pieces around". Wait long enough and have enough space, and eventually a human brain will appear just by random chance. It won't last long, but it can last long enough to have thoughts. Every other possible arrangement of atoms will happen as well. There is also a "Boltzmann flower pot" and so on.