X-ray crystallography is a core technique underpinning many important results in the field of biochemistry. Although most biochemists will not become experts in this technique, many will use the structural models deposited in the protein data bank for designing and interpreting other experiments. As there are a number of limitations to these models, it is important that undergraduate biochemists, as potential end-users of this technique, have some understanding of how these models are produced. A computer activity is described in which the program WinCoot is used to build part of a protein model to interpret an electron density map. This activity allows students to experience the nature of crystallographic data first-hand and discover how protein structural models are produced.