Some good advice for small friends in this book! If you're scared of something - in this case, a thunderstorm - think of something that makes you happy, and you'll feel a little less scared.
For the Very Little Ones, that will probably be enough. But from a parenting standpoint, it might take a little more explanation than "just think of something else," otherwise your small folks will only ever learn to control their fear without ever learning not to be afraid in the first place.
(Also, nothing will ever top Julie Andrews' version of this lesson with "My Favorite Things!")