Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_13640 ( "Stage" text, "Winner" text, "General classification" text, "Points classification" text, "Mountains classification" text, "Young rider classification" text )

### Question ###
What is the stage of winner Carlos Sastre when the general classification was Denis Menchov?

### Accurate SQL ###

SELECT "Stage" FROM table_13640 WHERE "General classification" = 'denis menchov' AND "Winner" = 'carlos sastre'