Mrs. Keenan, a preschool teacher, observed her 3-year-old granddaughter Riley pull, tug, and stack piles of holiday boxes on the floor. She remembered that her child care director had suggested using boxes as a curriculum theme, but she hadn't given much thought about the idea until now. She said to herself, "I wonder if my children would be as interested in boxes as Riley is. We could explore boxes as a theme for my class when we're back in school." How do teachers identify a compelling curriculum theme and then build on what children already know about it? This article shares the story of Mrs. Keenan, who realized that empty boxes are an abundant, inexpensive, and rich source for hands-on learning with 3- and 4-year-olds. (Contains 4 figures.)