What kind of material is the pen made of?
The pen is made of cement.
    src        edge_attr  dst
0     1   to the left of    2
1     1   to the left of    8
2     1   to the left of    5
3     1   to the left of    6
4     1   to the left of   16
5     1   to the left of   10
6     1   to the left of   17
7     1   to the left of   14
8     2  to the right of    1
9     2   to the left of   17
10    3  to the right of   17
11    3  to the right of    6
12    5   to the left of   15
13    5  to the right of    1
14    6  to the right of    1
15    6   to the left of    3
16    6   to the left of   17
17    8   to the left of   17
18    8  to the right of    1
19   10  to the right of    1
20   13  to the right of   15
21   14  to the right of    1
22   14   to the left of   15
23   14               on    5
24   15  to the right of    5
25   15  to the right of   14
26   15   to the left of   13
27   16  to the right of    1
28   17  to the right of    2
29   17  to the right of    1
30   17  to the right of    8
31   17  to the right of    6
32   17   to the left of    3
33   17               on    5