Teaching prime and composite numbers? Don't miss You Can Count on Monsters by Richard Evan Schwartz. In a work of creative genius (seriously, how do people think like this?) he combines art and prime/composite to form a series of monsters. His webpage gives you the best glimpse of how the monsters work...suffice it to say that prime monsters are frequently eaten by composites. The concept even captivated NPR with a story: Math Isn't So Scary with Help From These Monsters.
In my 5th grade math class, the book helped to level the playing field as some students were familiar with prime/composite and others weren't. As we read the book, the newbies grasped the basic concept and the old hands were captivated by the number trees. Longer explorations allow kids to make predictions about whether the next page will reveal a prime or composite number, including thoughts about what the number tree will look like.
Additional resources include:
A poster of 1-100, factored.
A fabulous prime/composite applet. Ask students to compare the applet to the information in the book.
The author has a series of videos explaining more about the book and its concepts.

