Need vs. Want

Need versus want - that's the question that's been kind of going through my head today, especially in regards to my girls.  How do I get them to understand that what they want most of the time isn't what they need?  For example, when we go to Target, L is constantly saying she needs to have this toy, this book, etc..  How do I tell her she doesn't need it, she just wants it?  I don't need to have this new shirt, but I definitely want it to put in my work rotation.  E doesn't need to have juice, she just wants it.  There are so many people in the world that don't even have their basic needs being met - how do I tell my daughters that they are so incredibly lucky not to have to worry about food, water, shelter, love, or air to breathe?  That they need those things, not the doll at the store their little hearts want.  Such a difficult topic.  I should just go to bed...