An IBM scientist holds a wafer that will get bonded with through-silicon vias, or TSVs, which are little connections that help spread out the task of transferring data from one point to another. First, the wafers are thinned so that more can be fit into a single package. Then, microscopic holes are drilled into the silicon. These get filled with metal (in this case tungsten). A layer of chips is then attached, sort of like that extra piece of bread on a peanut-butter sandwich.