Basic Science

Earn a bronze medal on all 18 Portal Challenges. After you beat the game, the Bonus Maps menu unlocks Challenge Maps: the six chambers 13-18, each with three targets - Least Portals, Least Steps and Least Time. Bronze thresholds are generous, so simply completing each challenge in a sensible way clears it. Knock out all 18 and you're well on the way to Rocket Science and Aperture Science too. Optional, post-game.