This is a clever book that begins with a problem I've not seen in many books. It is just before the beginning of school, and Evan is preparing for his 4th grade year. He is about to have an experience unlike what many kids have had-- his younger sister, Jessie, has been double-promoted from 2nd grade to 4th grade and will be in Evan's class. Jessie is obviously very intelligent, and while Evan is smart enough, he knows he is not at all the student Jessie is. 
 Evan and Jessie normally get along really well- better than most brothers and sisters, but now Evan is angry with his sister, even though he knows it isn't her fault. He can't even bring himself to spend time with her making a lemonade stand during the last weekend of summer vacation. This eventually leads to war as Jessie and Evan set up separate lemonade stands and fight to be the "best" at selling and making the most money. 
 This book is cleverly written and emphasizes economic and business concepts and math along with the story. This book has been popular with my fourth and fifth grade students, and I can see why. It keeps the reader's attention as we wait to see what tactic the kids will try next. This is the first book in a series.