CREATE TABLE table_25770 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" CLOB, "Directed by" CLOB, "Written by" CLOB, "Original air date" CLOB, "Production code" CLOB, "U.S. viewers  ( millions ) " CLOB)

What's the season number of the episode originally seen by 6.64 million people in the US?
SELECT MIN("No. in season") FROM table_25770 WHERE "U.S. viewers (millions)" = '6.64'
SELECT MIN("No. in season") FROM "table_25770" WHERE "U.S. viewers (millions)" = '6.64'