Even a four-year-old can sometimes stump a parent with "why" and "how" inquiries... kids love to ask questions and there are some great non- fiction picture books available that can help you answer them! One author, in particular, has written some very informative and interesting books on science, for kids aged 4-8.
Dr. Franklyn M. Branley (1915-2002) is the author of over 100 books about science for kids. He was Astronomer Emeritus and former Chairman of the American Museum-Hayden Planetarium. He was dedicated to explaining science and encouraging young children to explore their world through his series, Let's-Read-and-Find-Out Science, which he originated in the 1960's.
What I appreciated about Dr. Branley's books is that they are simple and well thought out - not dumbed-down, but not too complex. Some of the books have easy, hands-on experiments.
Here's a list of Franklyn M. Branleybooks available on Amazon.
NOTE: We've read a few of these and they were GREAT! Just make sure you preview them - not sure how he deals with evolutionary issues...