Why?

Delphine (still) asks us why a lot. Sadly, sometimes I don't have a good answer, which sort of led to the conversation we had this afternoon.

[First scene. Two cactuses are dancing.]
Delphine: "Why are the cactuses?"
Me: "Well, uh, they just are. Questions about why a thing is don't really have answers."

[Next scene. A goat has paint on himself.]
Delphine: "Why is he dirty?"
Me: "..."
Delphine: "He's dirty cause he was painting."

So, I guess some questions about why a thing is something are easy to answer but some questions about why a thing is something have no good answer. Can anyone tell me what the difference between the questions is, and how to explain it to a three-year-old without using words like "intrinsic", or "just because"?