Eh, 3rd graders can probably handle the area of a triangle, which can be viewed as a definite integral. (Somehow, middle school physics students seem to be abl

Eh, 3rd graders can probably handle the area of a triangle, which can be viewed as a definite integral. (Somehow, middle school physics students seem to be able to handle computing the distance traveled by an object given starting velocity and constant acceleration--again, technically an integral.) In fact, one can view a circle as being made up of a bunch of really thin triangles (this is how one usually derives the area of a circle), and this again is something that third-graders can appreciate--cut a paper circle into slices, and see if you can get the slices to look more and more like triangles as you cut them smaller. (I vaguely remember doing something like this in elementary school.)