Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_56 (thickness VARCHAR, diameter VARCHAR)
### Question:
Which Thickness has a Diameter of 21.9mm?
### Answer:
SELECT thickness FROM table_name_56 WHERE diameter = "21.9mm"