I often begin by having the students create triangles with a given perimeter, say 24 units. They are then to find as many Triangles as they can with this property. Almost inevitably, students draw a triangle and label the sides with something like 15, 5, 4. Afterwords, I have the students use a ruler to actually draw their triangles out. This is where they notice that they cannot. Finally, they are beginning to think about the fact that just because you label the triangle a certain way doesn't mean you can actually construct such a triangle. This applet allows you to take things a step further. You can explore several examples and really have the students think about the case in general. This should lead to them better understanding the triangle inequality.
