What is demand?
This question seems to be very easy, but it is the question which is mostly answered incorrectly.
Situation A: A man has $ 400 and he is willing to buy a CD worth $ 100 (Demand)
Situation B: A man has $100 and his willing to buy a CD worth $ 200 (Not a demand)
In order to constitute a demand and person should have both ability and willingness to buy a certain thing. In situation A the man had the ability and willingness to buy the thing, as he has $400 and the CD was only worth $ 100.
In Situation B the man was willing to buy the product, but he did not have the ability to buy it.