Reddit mentions of National Geographic Kids Why?: Over 1,111 Answers to Everything

Can't resist adding a few book suggestions - he's a bit young but I think you two would enjoy looking through something like David MacAulay's Way Things Work. A book of science questions and answers might be fun too, like How Come? or National Geographic's Why?; there's an equivalent for younger kids too but sounds like you guys might prefer the big kids version. Mistakes that Worked is another great book.