I recently tried to explain to my daughter (3 years old) that she was crying a lot because she was exhausted. The reason she was so tired was because she did not sleep after lunch. When asking her why she sang rather than trying to sleep, she told me "I did not sleep because I cry a lot". Obviously she does not understand the words because and why (not that I am surprised given her age). But, how can I explain these words to her? Or how can I ask my questions another way so she can give me relevant answers?
She understands the words just fine, what she doesn't understand is the underlying cause. Kids that age live very much in the moment. Past and present get jumbled in their mind, and they don't pause to reflect on their motivation.
Somehow parents can't resist asking why anyway, but I can count on one hand the number of times I've received an answer that makes sense to me. Just resign yourself to it being the parent's job to figure out why, and if you're lucky, your kids will confirm it for you.
You can explain the difference (if you must explain it) by saying Why asks the question and Because tells the answer.