What are the student ids of all who played Football?
select stuid from sportsinfo where sportname = "Football"
Which of those ids are also marked as having played Lacrosse?
select stuid from sportsinfo where sportname = "Football" intersect select stuid from sportsinfo where sportname = "Lacrosse"
What are the first names and ages that correspond to those student ids?
select fname, age from student where stuid in (select stuid from sportsinfo where sportname = "Football" intersect select stuid from sportsinfo where sportname = "Lacrosse")