Note that you can use your mouse to select any red point, called an anchor, and move it around. The lines connecting to the anchor will also move, but the remaining anchors will remain in place.

First, construct the midpoints on each side. Let A' be the midpoint on the side BC, let B' be the midpoint on the side CA, and let C' be the midpoint on the side AB.

Now connect each vertex to the midpoint of the opposite side to form the medians of the triangle. It appears that the three medians meet at a point G. In fact, they do, as we'll prove later.