Show the ids for all the students who participate in an activity and are under 20.
SELECT StuID FROM Participates_in INTERSECT SELECT StuID FROM Student WHERE age  <  20
SELECT StuID FROM Participates_in INTERSECT SELECT StuID FROM Student WHERE age  <  20
[{'stuid': 1001}, {'stuid': 1002}, {'stuid': 1006}, {'stuid': 1007}, {'stuid': 1009}, {'stuid': 1010}, {'stuid': 1014}, {'stuid': 1015}, {'stuid': 1016}, {'stuid': 1019}, {'stuid': 1021}, {'stuid': 1022}, {'stuid': 1024}, {'stuid': 1025}, {'stuid': 1029}, {'stuid': 1033}, {'stuid': 1034}]