CREATE TABLE table_21631 ("Series #" FLOAT, "Season #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT)

Which episodes did Nelson McCormick direct?
SELECT "Written by" FROM table_21631 WHERE "Directed by" = 'Nelson McCormick'
SELECT "Written by" FROM "table_21631" WHERE "Directed by" = 'Nelson McCormick'