Thinking in Physics (TIP) is a new curriculum that is more effective than commonly used interactive engagement methods for students who have the greatest difficulty learning physics. Research has shown a correlation between learning in physics and other factors, including scientific reasoning ability. The TIP curriculum addresses those factors. Features of the curriculum and evidence of its effectiveness are described. The most recent version of the TIP curriculum has greatly reduced a substantial gender gap that previously existed. More details and sample materials are provided in "Thinking in Physics," a book intended for instructors of introductory physics, published in 2014 by Pearson as part of its Educational Innovation series. Additional materials, both for students and instructors, are provided on the website http://thinkinginphysics.com. Both the book and the website are free.