Draw any line segment $\overline{PQ}$. Without measuring $\overline{PQ}$, construct a copy of $\overline{PQ}$.

Steps:

1. Draw any line segment PQ whose length is not known.

2. Place the pointer end of the compass at the point P and open the compass till the pencil end exactly coincides with the point Q. This opening of the compass gives the length of the segment PQ.

3. Draw any line l and take a point R on it. Without changing the opening of the compass, place the pointer end at the point R and draw an arc to cut the line l at the point S.

Hence, $\overline{RS}$ is a copy of the line segment $\overline{PQ}$.