An undergraduate first-semester general chemistry laboratory experiment introducing face-centered, body-centered, and simple cubic unit cells is presented. Emphasis is placed on the stacking arrangement of solid spheres used to produce a particular unit cell. Marbles and spherical magnets are employed to prepare each stacking arrangement. Packing efficiency is calculated using simple measurements of marble or magnet diameters and the dimensions of the stacking arrangement. Edge effects are introduced. Packing efficiency is subsequently calculated for various metal samples employing density measurements and literature atomic radii values. The stacking arrangement for each metal and its corresponding unit cell are determined by comparing packing efficiency values. Estimated percent unoccupied space values for water and air are also determined. (Contains 10 figures.)