Everything you need to understand or teach The Night the Heads Came by William Sleator.
In The Night the Heads Came, Sleator exercises his fertile imagination to conjure up visions of strange worlds, ancient ruined cities, bizarre creatures from other worlds, and an alien war being fought on Earth. The heads and the Others are locked in a cosmic contest with the people of Earth caught in the crossfire. Leo and Tim have seen wonders and learned cosmic secrets, and they are caught up in an ancient conflict...