Follow the steps below to earn the Schrodinger's Catch achievement.

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1) Load up chapter 6.

2) Play through chapter 6 until you enter a room and there will be a giant glass box with a cube in it.

3) Save your game once you enter this room.

4) Cover the cube inside the glass with repulsion gel by shooting a portal underneath the flowing repulsion gel.

5) When you hear the glass about to break shoot a portal at the water to wash the repulsion gel from the cube.

6) Repeat until one of the glass walls breaks.

7) Go to the broken glass panel.

8) Use the repulsion gel to enter the glass box.

9) Simply pick up the box and the achievement will pop up bagging you 20G.


Here is a video tutorial: