It's hot and steamy and Mom won't let Caillou go to the public pool since they've been every day this week! When Caillou refuses to put on sunscreen lotion and Rosie won't nap, Mom quickly realizes that the hot weather is making everyone grumpy. She and Caillou come up with the perfect solution: they connect the lawn sprinkler for some good-time backyard fun--which certainly doesn't need a lot of sophisticated equipment.