At 2.5, my DS found a bin that had 75+ hot wheels cars in it. He had a blast playing with them. After the usual zooming and exploring with them he sorted them all into seemingly random groups that he called friends. Different colors, different uses, trucks with cars, etc. After a little while we realized he was sorting by hubcap style.

At a store, he will tell you he would like such and such new car because it is friends with the airport police car and yellow race car... Still the same hubcaps.