David folds a sheet of paper. The dotted lines as shown in the figure are the creases formed, which are named l, m, and n.

What can you say about lines l and m?

Let's say that, a line n passes through line l and line m by making an angle x and y with line l and line m respectively.
From the figure, we can see that line n is acting as a transversal for lines l and m, and angle x and angle y are the corresponding angles.

From the figure, line n is perpendicular to line l and line m.
So, $\angle \mathrm{x} = \angle \mathrm{y} = 90° ... (\mathrm{Corresponding} \mathrm{angles})$

So, according to the concept of transversal, If transversal cuts two lines, and each pair of corresponding angles are equal in measure, then the respective lines are parallel to each other.

Thus, $\mathrm{l} \parallel \mathrm{m}.$