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_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?

### Accurate SQL ###

SELECT AVG(grid) FROM table_name_81 WHERE time_retired = "piston" AND laps < 15