Last week I tried a new concept of ' WHY '.It is very difficult to make a 3 yr old this concept.It is not something you can touch or feel.After a long thought , I tried asking her questions relating to her.Eg..Why do you want food?[when she asked me for it].She started to think and said..I don't know.I gave her the answer....Because I am hungry.Of course I wanted a full sentence each time.The same I tried with sleep, thirst,going to park etc, things she can relate too.It is also important that one uses it at the right time ,that is, at a time when she wants that particular thing, so that explaining is easy.I also use a lot of negative cross questioning.eg
You want to go to the park because you want to sleep?
she says at once...noooooo, because I want to go to the swing.
It works with most questions.A small child like that needs a lot of explaining but soon learns to understand the concept.