Remember the next concepts:
1) Orthocenter: It's the point where all three altitudes of the triangle intersect. An altitude is a line that passes through a vertex of the triangle and is perpendicular to the opposite side. In an obtuse triangle, the orthocenter is outside it.
2) Incenter: It is the point of intersection of all the three interior angle bisectors of the triangle. The bisectors always cross across the inner part of the triangle. Therefore, the incenter will always be located inside the triangle.
The incenter is the center of a circumference that is tangent to all the sides of the triangle, that's why the incenter has to be inside the triangle.
3) Centroid: It is the point of intersection of its medians (the lines joining each vertex with the midpoint of the opposite side). This point always lies inside the triangle
4) Circumcenter: The circumcenter is the center of a triangle's circumcircle.
In this case, the circumference that surrounds the triangle could have its center outside the inner part of the triangle. An obtuse triangle is one of these cases.