Imagine building a house out of rope. It wouldn't be easy. Neither is building individual molecules into rigid, hollow shapes. In fact, one of the most simple of molecular geometries-a stand-alone cube-has so far eluded scientists.

Now, Thomas Rauchfuss, a chemist at the University of Illinois at Urbana-Champaign, and his co-workers have constructed a molecular box that measures 5 angstroms on a side and 132 cubic angstroms in volume. Big enough to hold single atoms inside. The ability to trap atoms could one day enable the molecular box to function as a highly sensitive sensor.