This applet contains a Triangle with a Midsegment. You can move the vertices of the triangle and see how it effects the Midsegment. The length of the Midsegment and 3rd side are also displayed.

Point D is the Midpoint of AC, and Point E is the Midpoint of CB, which makes DE a Midsegment.
Move the vertices of the triangle until you think you could propose the Triangle Midsegment Theorem. There are 2 attributes that relate the Midsegment: DE to the 3rd side: AB.