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Narrative
Milk is poured into three petri dishes. Rennin, a protease, is added to two of the samples to curdle the milk. Calcium chloride is added to one of the samples.

Later, each petri dish is examined for extent of curdling. The first sample did not contain any rennin, and did not curdle. In the second dish, the rennin specifically cleaved a hydrophilic portion of the milk protein. This allowed a more hydrophobic portion of the protein to associate much closer, forming curds. The third sample, containing added calcium ions, has much larger curds since calcium ions form cross-links between the hydrophobic portions of the milk proteins.