We haven't been able to find any of C's sippy cups lately, so yesterday we went out and bought him several more. He spent the afternoon stacking them side by side, and then pushing one over and watching it knock the others as it fell, creating a domino effect. Kept him entertained forever. (Today I found seven of them buried in my car under toys and books, with various amounts of lumpy chocolate milk and fermented apple juice in them. EWW.)