A heavily plasticized PVC (PolyVinyl Chloride). The plasticizer agent in the vinyl makes it very soft and flexible. It also makes the vinyl form a sort of suction against smooth, flat surfaces, such as glass, polished metal, or gloss paint - much as a drop of water makes it difficult to separate two plates of glass. This is why ColorForms stick without adhesive. Wacky Wall Walkers make use of this property as well. If Wacky Wall Walkers were any more plasticized, they'd ooze down the wall as a true liquid. They'd have to call it Wacky Wall Slime.
Despite the name, the cling property of static vinyl is unrelated to static cling associated with static electricity.

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