Hello there!  Thank you for taking the time to work with me!  Can you provide me with a list of all of the different activity names?
select activity_name from activity
Can you filter that list to show the activity with the most student participants?
select t1.activity_name from activity as t1 join participates_in as t2 on t1.actid = t2.actid group by t1.actid order by count ( * ) desc limit 1
Great!  Can you tell me how many students in total participated in football?
select count ( distinct t2.stuid ) from activity as t1 join participates_in as t2 on t1.actid = t2.actid where t1.activity_name = 'Soccer'