CREATE TABLE table_3581 ("Season #" FLOAT, "Series #" FLOAT, "Episode title" TEXT, "Original air date" TEXT, "Nick prod. #" TEXT)

What is the series # on airdate September 21, 2002?
SELECT "Series #" FROM table_3581 WHERE "Original air date" = 'September 21, 2002'
SELECT "Series #" FROM "table_3581" WHERE "Original air date" = 'September 21, 2002'