Who was team 1 when the chalkheads were team 2?
CREATE TABLE "table1_18733480_1" (
  "episode_num" real,
  "air_date" text,
  "team_1" text,
  "players_left_after_round_1" text,
  "team_2" text
);
SELECT "team_1" FROM "table1_18733480_1" WHERE "team_2"='Chalkheads';