My son is 4, and he's constantly asking to do science experiments. We have a subscription to Koala Kits, and that's the high point of his month, but it's not enough. We've done the basics - baking soda and vinegar, milk/food colouring/dish soap, etc, but does anybody have any recommendations for subscription boxes, experiement kits, books, etc. that are geared for younger kids? Thanks!