What are the shelves made of?
The shelves are made of wood.
    src        edge_attr  dst
0     0   to the left of   16
1     0  to the right of   13
2     2  to the right of   13
3     2  to the right of    7
4     2  to the right of   10
..  ...              ...  ...
78   18   to the left of    9
79   19   to the left of   16
80   19  to the right of   20
81   20   to the left of   16
82   20   to the left of   19

[83 rows x 3 columns]