CREATE TABLE table_3158 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" CLOB, "Directed by" CLOB, "Written by" CLOB, "Original air date" CLOB, "U.S. viewers  ( millions ) " CLOB)

What was the airdate of episode number 18?
SELECT "Original air date" FROM table_3158 WHERE "No. in season" = '18'
SELECT "Original air date" FROM "table_3158" WHERE "No. in season" = '18'