It does a poor job of comparing a raven to a writing desk. It doesn't know they both have inky quills. Someone with an account should add that answer. ;)

It doesn't know enough about Dali's 'Persistence of Memory' or Botticelli's 'Birth of Venus' to compare them to da Vinci's 'Mona Lisa', but if you ask it to compare the Dali and Botticelli with each other it does a good job of extracting data from Wikipedia to compare them to each other. http://www.diffen.com/difference/Birth_Of_Venus_vs_Persisten...

It was fun to try out a few things. One of the poorest matches in my limited tries was chlorophyll vs. xanthophyll. http://www.diffen.com/difference/Chlorophyll_vs_Xanthophyll . It found nothing for the latter and one of the little bits it know about chlorophyll is 'types: many such as a, b, c etc.'