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_81 (grid INTEGER, time_retired VARCHAR, laps VARCHAR)
### Question:
what's the average grid that has a time/retired piston and laps smaller than 15?
### Answer:
SELECT AVG(grid) FROM table_name_81 WHERE time_retired = "piston" AND laps < 15