Letter locking - BBC article on historic method of keeping secrets in mail.

YC: Music rise after netbook for every child - Children were given netbooks through their school with limitations, but it allowed them the freedom of having their own computer versus the norm of having a family shared computer, or none at all.

YC: TDD at Factoria - Creator of Factorio became aware of the videos of Uncle Bob and began implementing some code factoring that ended up being a truly great addition to their software arsenal.

YC: Liquidity of skill - On the subject of “It doesn't matter how good you are at something, if no one actually know you are good at it.”