Explaining Why

We have been trying to teach our 3-year-old Sydney what the word "why" means. When we ask her "Why didn't you go potty in the potty seat?" or "Why did you fill up the toilet with baby wipes?" She just gives us this blank stare. Sometimes she answers the question like we're asking her where she did things. We've tried to get her to fill in the blank -- "I did this because..." I guess this is one of those things that they just have to catch on to naturally. Defining the word why just doesn't work too well. I don't even know how to do it without using bigger words that she also doesn't know. I'm sure once she figures it out she'll be asking me why all day long. I'd like to learn her reasoning on some of the random things she does though.