Praise from the marvellous Dooce:

Many readers have sent me books to help with explaining to Leta how the baby got inside my tummy, and this book, Where You Came From by Sara O'Leary, is one of my favorites. Inside there are no cows humping other cows or pictures of sperm swimming toward a woman's ovaries, and instead there are playful illustrations depicting possible explanations, like a baby in a sale bin at the grocery store or parachuting from a spaceship. I love the answer at the end, and I won't give it away, but I think it's perfect for beginning the discussion with a four-year-old.