user: Question: What number episode in the series was watched by 15.89 million U.S. viewers? 
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original airdate', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [71.0, 1.0, '"Shalom"', 'William Webb', 'Donald P. Bellisario and John Kelley', 'September19,2006', '13.80']
SQL:

assistant: SELECT "No. in series" FROM "Table" WHERE "U.S. viewers (millions)" = '15.89';