Evan and his sister Jessie usually get along just fine—until he finds out that she's skipping third grade. That means she'll be in his fourth grade class next year, showing him up every day with her perfect grades. Now Evan doesn't want her help with his lemonade stand, even though he could really use her math skills to make a profit.

Jessie, who sometimes finds people more puzzling than math problems, doesn't understand why Evan won't team up with her, so she starts her own lemonade stand. Soon brother and sister are waging an all-out war for customers. Can Evan figure out the math to make more than he's spending? And can Jessie attract customers and navigate a new friendship without Evan's help?

Told in the alternating voices of Jessie and Evan, and including kid-friendly business advice, this believable story illustrates the grudges and misunderstandings—and unconditional support—that make sibling relationships unique.