Your instinct could be the subject.

Human beings tend to compare the subjects instinctively.

Hierarchical relation is the most common thing that I think.

How much your income is or which university you graduated from are one of the things. In Japan, what floor you are now living in can be the subject because the higher floor indirectly reflects the higher income.

That is why I hope you to utilize this instinct for your constructive educational purpose.

For the next target of iPhone APP Catch Questions, the new function will allow you to compare your questions based on some factors. It is necessary for your neural circuit in the permanent storage of your brain to be activated so that you can remember the subject efficiently.

To do study smart, think of your instinct and understand it!