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