Problem 3. Let the cube $A B C D E F G H$ be given. How many segments are there whose endpoints are the vertices of the cube? Explain your answer.
Solution. The cube has 8 vertices. From each vertex, we can draw 7 segments to the other vertices. In this way, we get $8 \cdot 7=56$ segments. However, in the counting, each segment appears 2 times. Therefore, there are $56: 2=28$ segments.