Pinacol, $(\mathrm{CH}{}_3){}_2\mathrm{C}(\mathrm{OH})-\mathrm{C}(\mathrm{OH})(\mathrm{CH}{}_3){}_2$, is warmed with dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$. The main organic product is

Protonation and loss of water from one $\mathrm{OH}$ gives a tertiary cation. A methyl group migrates from the neighbouring carbinol carbon, giving an oxygen-stabilised cation. Loss of $\mathrm{H}{}^{+}$ from this protonated ketone gives pinacolone, 3,3-dimethylbutan-2-one.