Convex vs. Concave

In the app below, move the vertices of the shown polygon around. Be sure to explore the triangle, quadrilateral, and pentagon. Then, answer the questions that follow.

According to the app, how would you describe what it means for a polygon to be convex? How would you describe what it means for a polygon to be concave?

Is it ever possible for a triangle to be concave? Why or why not?