How can you break a puzzle this simple? Quite easily, actually! As always, it involves doing silly things with the cube.
How do I accomplish that?
Place a portal near the emancipation grid in front of the exit elevator, then return to the cube. You'll need to destroy the cube to trigger the unique dialogue, so carry (or throw) it through another portal so it makes contact with the grid.
Note that the dialogue will only play when the cube is destroyed. If it lands in the corridor and your portal in there is moved, you'll be stuck and need to restart!
What is the unique dialogue?
"Please do not attempt to remove testing apparatus from the testing area. A replacement Aperture Science Weighted Storage Cube will be delivered shortly."
Thanks to YouTuber Marblr; I found this softlock through their excellent "Every Softlock in Portal" video!
Guide help
This is the low-spoiler guide for "Breaking the puzzle" puzzle in Portal. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.
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