Okay kiddo, have you heard of elephants before? They're those big grey animals with trunks that you might have seen in pictures or at the zoo. People have known about elephants for a really long time, and a long, long time ago, some people in Egypt used to write on pieces of paper made out of a plant called papyrus.
When they needed more paper to write on, they would sometimes use the tough skin of the elephant to make it. This kind of paper made from elephant skin is called "elephantine papyri." It's like regular paper, but tougher and more durable because it's made from a strong animal like the elephant.
Now, imagine that you're in Egypt and you want to write something important down, but you don't have any elephantine papyri around. You might look for something else to write on, like a piece of pottery or maybe even a smooth rock. We call these sorts of things "ostraca."
Ostraca are like small blank canvases that people can use to write messages or create art. Since they're made out of things like pottery or stone, they can last a really long time and give us a glimpse into what life was like in ancient Egypt.
So, to sum it up, elephantine papyri are pieces of paper made from the skin of elephants that people in ancient Egypt used to write on, while ostraca are small objects that people used like notebooks to write on when they didn't have any papyrus or elephantine papyri.