comes from college campuses that have enough room and enough hills and good winters....
traying is like sledding, only on a cafeteria tray.... we college kids don't have room to keep a sled in our rooms, so we "borrow" a lunch tray and we're set.... better than snow disks!
a little known urban sport which involves a challenge between 2 or more friends as to who can bin the most take away trays (as in the ones provided at mcdonalds) as possible at once.