An easy, percipient third-grade story in the Cleary mode. It begins, on the first day of school, with Chris wanting a cat but knowing he can't have one, for ""50 different reasons,"" as his mother says. Then, however, when his photographer father receives a long overdue payment, his buoyant mother takes him out to buy a cat--a Domestic, which sounds impressive to Chris until Veronica across the hall, a child who always has one better than yours whatever it is, tells him that's the same as a plain old alley cat. But Chris is happy with Tiger--though at school he goes through an empathically described period of jealousy, uncharacteristic clowning, guilt, and estrangement from his best friend Peter, all set off when Peter is assigned to the top reading group. The story builds to a grand triumph when Chris' father gets a dollhouse ad job and shoots it in their apartment, with all sorts of support people scrambling around and two little girls as models. Then, next morning, Tiger gets into the dark room and messes up the negatives. Robinson softens the disaster without making everything okay by having Chris' father find one negative that's ""hot too bad."" Appealing, often funny, with a feel for a third-grader's experiences.