In Minecraft's 2011 Beta code (Beta 1.7.3), every block holds a light level from 0 to 15. A dark cave is 0, and a torch gives 14 (15 times 0.9375 in the code). A real flame lights a round pool, but walk away from a torch on a diagonal and the dark comes twice as fast.

The reason is counting. In the game's own light code, each block looks at its six neighbors, takes the brightest, and subtracts one. So light travels in straight steps: 14, 13, 12. Every block the same number of steps away gets the same light, and those blocks make a diamond. A diagonal is two steps, so it loses two. A solid block is set to 0, so a wall stops it, and with two torches each block keeps the brighter count. Fourteen steps out, it hits zero.

The floor, the wall, the torch and the light run in this film are our own simplified version, built in code; the rule is the game's own (Beta 1.7.3, read in the community decompile Project Poseidon).

Which block-world rule should we cut open next?

Sources: Minecraft Beta 1.7.3 server code, decompiled (Project Poseidon on GitHub): Block.java and MetadataChunkBlock.java.
Not an official Minecraft video. Not approved by or associated with Mojang or Microsoft.
Sounds: our own, made in code.