The Wonderful Flight to the Mushroom Planet by Eleanor Cameron

I first met this book, tagging along with my mother to yard sales of a Saturday morning. It was in a box of books in my price range - presumably ten cents or so - an proved to be one of those rare and delightful finds which I enjoy revisiting every few years, and am now reading to my children.
Science fiction from the time when science was definitely cool, this is the story of a boy (two boys, really- David and his best friend, Chuck) who answer an ad in the newspaper to design and build a boy-sized spaceship. The reward, as described in the ad, is the chance for an adventure and to do a good deed.
I don't want to spoil it, so I won't say more about the plot. It's good enough to read aloud; the science is very cleverly done, so that it remains... if not likely, within the realm of the imaginable. From a homeschooling perspective, this is a wonderful launch pad for a discussion about what is possible, what is not, and what we just don't know with regards to space, space travel, other planets, earthly and other biology.