This achievement is earned for finding optional photo #9. If missed on the first run, you can "Play Episodes" and select specific parts of the Episode in order to find the missing optional photos through collectibles mode
. The following collectible is described and can be seen below:
] - Optional Photo #9
Inside the school, go to the chemistry room, talk to Warren, rewind, talk the to teacher and ask about the experiment. Return to Warren and recommend Chlorine. take a picture of the successful experiment afterwards.
Collecting all 10 photos will also reward you with the following achievement: