Win a gold medal on a challenge map.
Once you have control of both portals, only enter the orange portal.
Find all the Rat Man dens.
Complete the game without ever taking a bullet.
Cause a rocket sentry to destroy its own rocket when the rocket has been redirected back towards it.
Knock a turret over with a security camera.
Trap yourself in a test chamber so that GLaDOS has to help you continue.
Dissolve a turret.
Find the chicken's test results.
Beat 5 advanced chambers
Beat 10 advanced chambers
Beat all advanced chambers