3. Classic Gyroscope
Ages 5 and up
Surely what Barbie meant to say is that physics class is tough. So we applaud any toy that establishes the bona fide awesomeness of the subject, toughness and all. Turns out you can't beat the gyroscope for a fascinating lesson in angular momentum -- the same mysterious force that keeps bicycles upright, ships on course, and satellites orbiting the planet. The gyroscope has been packaging this lesson into a sturdy, handy, and portable device for roughly a century, and it's still inexpensive and mind-blowing after all these years. Yank the string, and the spinning wheel will defy gravity long enough to inspire an interest in angular velocity, nutation, torque vectors, and other physics staples. Your kid will have an innate grasp of conservation of energy years before it shows up on a problem set.
The bottom line: Teach physics with a centuries-old classic.