In this tic-tac-toe variant played with all X's, the loser is actually the winner

In Misère Tic-tac-toe, getting three-in-a-row means you lose. I can’t wait to unleash this game on my kids at our next dinner out.

Here is a wonderfully-named app version: Notakto

Here is a scientific paper on the mathematics of the game: “The Secrets of Notakto: Winning at X-onlyTic-Tac-Toe” by Thane E. Plambeck, Greg Whitehead

And below is part 2 of the Numberphile video. Note from the video description:

Correction from Thane: The configuration with two X’s, one in a corner and one in the middle, is a “b” and not a “b^2”. It’s at 5:24 in the video.

(via Cliff Pickover)