Why, its like comparing two things from the same category!

"That's like comparing apples and oranges."

Its a statement meant to convey the idea that the two things you are discussing are can't be compared. That subect/item A is so different from subject/item B that any analysis or comparisson is rendered irrelevant or moot.

But the use of this line is one of my pet peeves (or annoyingly overthought ideas, take your pick) and I take some pride in being one of the few who oppose its use. Because rephrased, the speaker is saying "that's like comparing two fruits."

And why couldn't you compare two fruits? When you go the grocery store, don't you decide between purchasing apples or oranges? If you were going to the store to buy food, you'd compare apples and oranges as options, where as you wouldn't compare apples and say cereal, or milk, or bread. So you can compare apples and oranges at the macro level of food. But you can also compare them at the micro level of fruit.

In actuality, unless you go so micro as to include only "apples" or strictly "oranges" as your category, apples and oranges are entirely comparable.

Its like comparing baseball and football as sports. Both are games, both are sports. Comparable. Sure, the NFL and AFL versions are both football, but that doesn't exclude a comparison between football and baseball. Now, you would never compare football to say house cleaning, or writing, or painting.