When a glass of wine is oscillated horizontally at 4Hz, the liquid surface oscillates calmly. But when the same amount of liquid is contained in a cylindrical mug and oscillated under the same conditions, the liquid starts to oscillate aggressively against the container walls and results in significant spillage. This is a manifestation of the same principles that also cause coffee spillage when we walk. In this study, we experimentally investigate the cup motion and liquid oscillation during locomotion. The frequency spectrum of each motion reveals that the second harmonic mode of the hand motion corresponds to the resonance frequency of the first antisymmetric mode of coffee oscillation, resulting in maximum spillage. By applying these experimental findings, a number of methods to suppress resonance are presented. Then, we construct two mechanical models to rationalize our experimental findings and gain further insight; both models successfully predict actual hand behaviors.