Machine clocks drift, so timestamps can't tell you which of two events in a distributed system came first. Logical clocks can — Lamport clocks give a consistent order, vector clocks tell you whether an order exists at all.

The key prop isn't there to silence a console warning. It's how React decides whether an element is the same thing it rendered last time — and getting it wrong moves state to the wrong row. How the diff works, and how to pick a key.

Spread and Object.assign copy one level; everything below is shared. Why that causes “I copied it and the original still changed”, how structuredClone fixes it, and the functions, classes and DOM nodes it refuses to touch.

Models don't see characters or words — they see tokens, produced by byte-pair encoding. How BPE learns its vocabulary, why it explains so many odd LLM behaviours, and what it means for cost and context limits.