Model Building by Watson & Crick
 Watson and Crick could form hypotheses about
DNA's structure by building a physical model of
how its atoms fit together
They had the machine shop of the Cavendish Lab 
at Cambridge University make copies of nucleotide
and fit them together via their chemical properties.

Watson & Crick
 Figure 16.1 on page 314 of Campbell Bio 11e  Fig 13.6 on page 272 of Life by Sadava et al 11e