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.