Plastic
Plastic is an artificial substance created on Earth by Humans to make such items as maniquinns, phones, cables, dolls, inflatable chairs, artificial flowers, and breast implants. Plastic has no real value to beings unlike gold or silver does.
The Nestenes are a race of cephalpods able to animate anything made of plastic, giving them the identification of living plastic creatures. Their creations, the Autons, are their best known creatures.(DW: Spearhead from Space, Rose) Usually, the items they bring to life must be solid plastic.(DW: Terror of the Autons)
When up agaist bullets, only at close range can they penetrate and blow plastic apart.(DW: Terror of the Autons)