2,2-Dimethyloxirane (isobutylene oxide) is warmed with methanol containing a few drops of $\mathrm{H}{}_2\mathrm{SO}{}_4$. The major product is

Acid protonates the epoxide oxygen and the $\mathrm{C}-\mathrm{O}$ bond to the tertiary carbon is weakened, so that carbon carries considerable positive charge. Methanol attacks this more substituted carbon. The oxygen of the original epoxide ends up as the primary alcohol, so the product is 2-methoxy-2-methylpropan-1-ol.