Chemists have tied the tightest knot yet, a nano-sized structure with eight crossings and just 192 atoms. The advance could help researchers learn how to manipulate materials at the atom level to develop stronger, more flexible, and lighter-weight cloth or construction materials. The knot, described in today's issue of the journal Science, measures 20 nanometers in length, about 100,000 times smaller than the head of a pin. Why make a knot that's so small? I'll give you a two-part answer.