Write SQL query to solve given problem: According to all the observations of Elly Koss, what was her average weight?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT AVG(T2.VALUE), T2.units FROM patients AS T1 INNER JOIN observations AS T2 ON T1.patient = T2.PATIENT WHERE T1.first = 'Elly' AND T1.last = 'Koss' AND T2.description = 'Body Weight'