"Why, Daddy?" is easily the most gratifying and frustrating question a child can ask, as any parent is (or soon will be) aware. As recent Geekdad posts have made clear, there are tons of resources out there for answering basic kid-type questions. The one I remember from my childhood is Arkady Leokum’s Tell Me Why series, copies of which are still easy to find, and still useful in answering questions of basic science.
Leokum wrote a whole series of Tell Me Why books in the 60s, starting with Tell Me Why, and moving through such descriptive, if uninspiring titles as More Tell Me Why, Still More Tell Me Why, Yet Another Tell Me Why and so on. In the mid-90s, these books were republished (and, I believe, updated) in compendium format, three books to a volume, under the name The Big Book of Tell Me Why.
Don’t let the age of the books fool you. Though they’re are out of print, all the titles — especially the 90s editions — are still readily available from used booksellers. I recently bought a copy of the first updated volume recently, which answers basic questions ranging from "Why does the moon shine?" to "Why does soap clean?" to "How do spiders spin their webs?" It’s just as sharp as I remember it. Certainly some few of the answers might be slightly out of date, but the basic science and history in the books, along with their simple illustrations and clear, concise, older-kid-friendly writing will never go stale. I suggest copies for all geekdads.