Reaction between acetone and methylmagnesium chloride followed by hydrolysis will give:

Acetone (a ketone) reacts with a Grignard reagent to form an adduct which upon hydrolysis yields a tertiary alcohol.
$$C{H}_{3}COC{H}_3+C{H}_{3}MgCl\to (C{H}_3{)}_{3}C-OMgCl\stackrel{H_{2}O}{\to }(C{H}_3{)}_{3}C-OH+Mg(OH)Cl$$
The product is 2-methylpropan-2-ol, commonly known as tert-butyl alcohol.