[basementcat]

+1 on looking at words.

I once was having dinner with a girl and the topic of conversation drifted to dreams and how one might tell if one is in an "Inception" type scenario. I used the menu as an example-- read a line from the menu, close the booklet, reopen and reread the same line. I told her if the two lines aren't the same line, you're dreaming (the rendering hardware in your brain apparently uses different PRNG seeds for procedural texture generation).

I paused awkwardly as I realized that items in the menu kept changing each time I reread them.

She was very offended when I told her she was a figment of my imagination.

[veli_joza]

I like your explanation for changing lines of text, but I don't think brain is actively generating any content. I think brain is just trying its best to apply daytime pattern recognition to un-orchestrated neural activity. The most recently learned patterns are the strongest, which is why often you'll dream stuff that happened that day. If you spend a lot of time performing single activity (like gaming), the dream will be intensive and focused; this is called the Tetris effect.