Linus Torvalds, then a Finnish student, announced Linux on 25 August 1991.

He posted to the comp.os.minix newsgroup with the now-famous understatement: "I'm doing a (free) operating system (just a hobby, won't be big and professional)." The context matters: real Unix ran on expensive university machines, and the only thing close to it for a home PC was MINIX, a teaching OS he found too limited and too restrictively licensed to build on.

So his goal was concrete and personal — get a Unix-like system running on his own i386 PC, free for anyone to use and modify. That "free and modifiable" choice is what turned a hobby into a global project: thousands of developers could pile in, which a single hobbyist could never have matched.

• The name = "Linus' Unix" (originally he wanted to call it Freax).
• It began as one person's side project and now runs most servers, all the top supercomputers, and (via Android) most phones.

Tip: Anchor the date as "91." GNU started in 83, the GPL in 89, Linux in 91 — the building blocks lined up over a single decade.

Go deeper:

• History of Linux (Wikipedia) — Torvalds' 25 Aug 1991 comp.os.minix announcement and the full milestone timeline.