First, you're gonna want to be in the crazy box level from chapter 6. Go ahead and pause the game once you're at this chamber, and save your game so if you mess something up, you can load back into the chamber.

Make a portal where the blue gel is, and another on top of the cube. The cube will start bouncing around like crazy (crazy box, lol). Once the glass has broken, and the cube is still in the box, change the portal that was under the blue gel out and put it where the water is. Go inside the box, pick up the cube, and you should get the achievement!

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Thanks for reading, and I hope you have a nice™ day :D