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_23840 ( "Type" text, "Beam height (mm)" real, "Flange width (mm)" real, "Web thickness (mm)" text, "Flange thickness (mm)" text, "Weight (kg/m)" text, "Cross-section area (cm 2 )" text, "Moment of inertia in torsion (J) (cm 4 )" text )

### Question ###
What is the flange thickness (mm) for the weight (kg/m) 10.4?

### Accurate SQL ###

SELECT "Flange thickness (mm)" FROM table_23840 WHERE "Weight (kg/m)" = '10.4'