There are three levels of comprehension:

• Behavioral- lower level processing, basic ABC's
• Cognitive- higher level processing, math and science
• Constructive- highest level of processing, abstract problem solving

I believe that Joey demonstrates a level of comprehension of behavioral at least when it comes to school related things and common sense things. He struggles with math and any concept of science. His common sense is also very shaking.

Examples:
Joey's Card Trick

Joey's 3D glasses

Joey's hiding place