user: Question: Tell me the surface for 16 April 2007
Columns: ['Outcome', 'Date', 'Tournament', 'Surface', 'Partner', 'Opponents in the final', 'Score in the final']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Runner-up', '21 July 2003', 'Umag, Croatia', 'Clay', 'Thomas Shimada', 'Álex López Morón Rafael Nadal', '1–6, 3–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Date" = '16 April 2007';