Gravity. I knew that funny looking hair would be useful in remembering this!
So the apple has grown on the tree. Hanging there, the apple has potential energy, because it is still, but it can fall due to...?
So now, the stem can no longer support the weight of the apple. The stem breaks, the apple drops- converting the potential energy to kinetic- and you just so happened to be sitting under it when this occurred.
Thanks Mr. Robert! We'll see you tomorrow!
You're plenty welcome, kids. Don't forget to do your homework!