This is a tutorial explaining how to unlock the achievement Preservation of Mass.


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1) Load up Chapter 2, Test Chamber 7.

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2)Play through the chamber until you can leave through the exit door.

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3) Walk through the door.

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4) Place a portal on the wall opposite.

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5) Go back into the test chamber.

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6) Place a Portal wherever possible.

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7) Pick up a cube.

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8) Head through the portal you placed in the main test chamber.

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9) Enter the lift to enter test chamber 8.

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10) The achievement will pop up earning you 20G.

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Here is a video tutorial: