One day while watching her cartoons on tv, some advertisements came on.
My Girl: Mummy, what happened to my show? Finished already?
MG: No, that is an advertisement. An advertisement shows you something inviting you to buy it. When you see the advertisement then sometimes it makes you want to buy it.
To illustrate the point, each time an advertisement comes on, we would ask her to think what the advertisement is about ie inviting people to buy what? She would reply "wash hair" for shampoo etc.
A few weeks later......
My Girl: Mummy, I want to buy the barbie dollhouse I saw on the advertisement on tv. I want to buy!
MG: *slaps forehead*