wearing(person1, hat), is_hat(hat), is_green(hat), is_person(person1), is_person(person2), looking_at(person1, person2).
I wanted to apply this to Brazilian Jiu Jitsu videos to be able to find very specific queries like, "matches where player 1 gets a takedown, gets swept by player 2, and player 2 wins by submission". A sufficiently well tagged data set would let you find specific stories and sequences of events in a way that I don't think a non-computational query system could do.