Fuck off. You can't just say things like that without telling the class what the difference is.
Edit: it looks like the right one has white specks and the left one doesn't? Can't tell if the white specks are actually there or just image compression or different lighting though.
You are right, my bad. :) One is granite and the other one is granodiorite.
Granite is a rock with certain minerals in certain ratios: quartz, alkali feldspar(AF) and plagioclase(PL), where there is more AF than PL.(+accessory minerals of course but let's keep it simple). Granodiorite has more PL than AF. For more information, look up the Streckeisen diagram, it's a little scary at first but there are good explanations of it. :)
We were allowed to use hydrochloric acid to help differentiate rocks that one time we had geology as a subject in geography class. Maybe that helps finding out the difference here?