What's the production code of the episode originally aired on February 21, 1985?
CREATE TABLE "table1_2818164_2" (
  "no_in_series" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" real
);
SELECT MIN("production_code") FROM "table1_2818164_2" WHERE "original_air_date"='February 21, 1985';