Here is an interesting expression in Spanish:
¡Estás mezclando churras con merinas!
(You are mixing churras and merinas!)
It is (more or less) equivalent to the English expression mixing (or comparing) apples and oranges: we use it when we want to let a person know that he or she is comparing two items or topics that are incommensurable.
So what is a churra and what is a merina?
This idiomatic expression is mostly used in Spain, as churras and merinas happen to be the two most prevalent types of sheep in the country.
Here is a picture of some churras:
And here are some merinas:
As we can see, they are obviously very different types of sheep, and thus we should never try to compare them :)