2,3-Dimethylbutane-2,3-diol (pinacol) is treated with $\mathrm{HIO}{}_4$. The product is

The C2-C3 bond between the two tertiary carbinol carbons is cleaved. Each carbon carries two methyl groups, so each fragment becomes $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{O}$, giving two molecules of propanone.