CREATE TABLE table_26432 ("No. by series" FLOAT, "No. by season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production number" FLOAT, "U.S. viewers  ( in millions ) " TEXT)

Who directed episode 21?
SELECT "Directed by" FROM table_26432 WHERE "No. by season" = '21'
SELECT "Directed by" FROM "table_26432" WHERE "No. by season" = '21'