The Blob.

A drive-in favorite, this sci-fi classic follows teenagers Steve and his best girl, Jane, as they try to protect their hometown from a gelatinous alien life form that engulfs everything it touches. The first to discover the substance and live to tell about it, Steve and Jane witness the blob destroying an elderly man, then it growing to a terrifying size. But no one else has seen the goo, and policeman Dave refuses to believe the kids without proof.