How to Think Like a Scientist: Answering Questions by the Scientific Method - Stephen P. Kramer, Felicia Bond
This is a really fantastic little book to introduce both the scientific method as well as critical thinking skills. The first chapter was my favorite as it introduces the following ideas - (1) information can be observed correctly, but easily interpreted incorrectly, (2) be wary of accepting other people's conclusions (even so-called experts!), and (3) my personal favorite - how our own desires can affect the conclusions we reach - i.e. just because you want it to be so doesn't make it so.

We had fun applying all of this to my daughter's recent discovery about the truth about Santa. We went through her pre-truth reasoning: The experts (i.e. my parents) tell me Santa is real, I observe that the presents 'magically' appear on Christmas morning, and I desperately *want* Santa to be real - therefore he *must* be real! And then her post-truth reality: The experts (i.e. my parents) had a vested interest in perpetuating the Santa myth (it's fun!), the presents were indeed there, but were not placed by Santa and, finally, just because I *wanted* Santa to be real does not negate the laws of space and time.

Anyway, the book goes on to explain the five steps of the Scientific Method - perhaps not the most riveting reading, but still nicely done. And, finally, encourages children to *constantly* ask questions and apply the Scientific Method to find the answers.