Database: olympics
Question: What is the average weight of the competitors who won a silver medal?
SQL: SELECT AVG(T1.weight) FROM person AS T1 INNER JOIN games_competitor AS T2 ON T1.id = T2.person_id INNER JOIN competitor_event AS T3 ON T2.id = T3.competitor_id INNER JOIN medal AS T4 ON T3.medal_id = T4.id WHERE T4.medal_name = 'Silver'