What are the shelves made of?
The shelves are made of metal.
    src        edge_attr  dst
0     0  to the right of    1
1     0  to the right of    6
2     0  to the right of   12
3     0  to the right of   15
4     0  to the right of    9
..  ...              ...  ...
80   18   to the left of   13
81   18   to the left of   14
82   18   to the left of   19
83   19  to the right of   18
84   19   to the left of    9

[85 rows x 3 columns]