(1-Methylcyclobutyl)methanol, $(\mathrm{CH}{}_2){}_3\mathrm{C}(\mathrm{CH}{}_3)-\mathrm{CH}{}_2\mathrm{OH}$ (a cyclobutane ring bearing a methyl and a hydroxymethyl on the same carbon), is heated with conc. $\mathrm{H}{}_2\mathrm{SO}{}_4$. The major alkene is

Loss of water directly would give an unstable primary cation, and the adjacent ring carbon has no hydrogen, so no alkene can form without rearrangement. A ring C-C bond migrates to the carbocation centre, expanding the strained four-membered ring to a five-membered ring and giving the tertiary 1-methylcyclopentyl cation. Loss of a proton gives the most substituted alkene, 1-methylcyclopentene.