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 [color=#ff7700][b]convex[/b][/color]? How would you describe what it means for a polygon to be [color=#1e84cc][b]concave[/b][/color]?
Is it ever possible for a triangle to be [color=#1e84cc][b]concave[/b][/color]? Why or why not?
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Information: Convex vs. Concave