Lex has a harder time remembering that tomatoes are a fruit than he does apples. this is because apples more closely resemble his of fruit.

They resemble a schema of fruit more closely. The apple tends to have the stereotypical "sweet" flavor and texture that one would align with the fruit schema. Our mental representations of an object or category can, such as in this case, make it easier or tougher to align new information with these already-existing categories.