Question 2

Folding a line segment onto itself creates a crease perpendicular to that segment.

Step 1 — Understanding the goal

Let us take a paper triangle. We name its vertices A, B, and C. Let us choose side BC as the base. Vertex A is the top vertex. We want to fold an altitude from A. An altitude is a line from a vertex. It meets the opposite side at a right angle. So, the crease must be perpendicular to BC.

Step 2 — How to make a perpendicular fold

We need the crease to be perpendicular to base BC. To make a fold perpendicular to a line, we do this. We fold the paper so line BC lies perfectly on itself. The edge BC matches up exactly. Think about this fold. The crease line acts like a mirror. Any point on BC on one side of the crease. It would land on a point on BC on the other side. But BC folds onto itself. So, every point on BC lands on itself. This means the crease must form a $90^\circ$ angle with BC. If not, line BC would not perfectly align. So, the crease is perpendicular to base BC.

Step 3 — Completing the altitude fold

We now have a fold perpendicular to BC. We must also make this crease pass through vertex A. We adjust the paper while folding. We keep base BC aligned with itself. We move the paper until the crease goes through A. The point where the crease touches BC is the foot. This crease passes through A. It is also perpendicular to BC. So, this crease is the altitude from A. Therefore, the crease formed is perpendicular to the base.

Answer

The crease formed is perpendicular to the base because the folding action aligns the base line perfectly onto itself, which inherently creates a fold line perpendicular to it.