Supply and demand, my dear,"" says Millie's Dad when he cuts off her allowance so she will learn to stand on her own feet. ""Find something that's scarce--something that's hard to get--but something that people want. And then supply it."" And so Millie is off. When she learns from little brother Willie that the fourth grade boys are collecting horse chestnuts, she offers to buy them all for a nickel apiece, figuring the resultant scarcity will make them worth a dime. The kids' derisive skepticism only makes her more determined, and soon she has friends Rose Ann and Gwenny selling her chestnuts for 15Â¢ while she, to ensure that they move, buys them back from her friends' customers for 20Â¢ Friendly Mr. Brinker at the drug store agrees to take some on consignment, and Millie goes on buying them back so he'll see that they sell and buy her out. When Millie forms a corporation and brings other kids in as investors, Millie's mother orders a stop; but her father joins the corporation--and responds to Millie's dawning qualms (""Is it good to make things that used to be free worth money?"") with a slightly nervous ""It's the way the system works."" She has serious second thoughts when Mr. Brinker, now buying for 50Â¢, gets wind of her ""scare"" and asks, ""Millie, you weren't trying to set me up, were you?"" So what to do when used-car dealer Mr. Gottlieb offers to buy her entire stock of 12,000 chestnuts for a dollar a piece? Dad and the kids vote to sell, but Millie can't quite bring herself to sign. . . until, this being essentially a comedy, Mr. Gottlieb lets her off the hook by explaining what he's up to. Despite the profitable outcome, Millie has lost her taste for business and learned too that Dad, though lovable, is not always right--a refreshing admission in children's books. The whole story, like Hughes' Honestly, Myron and Nutty for President, is a breezy, laugh-coated lesson in which the values find their way to the surface without preaching or contrivance.