What properties do both glass and plastic contain that allows them to substitute for each other?

Question

Chemistry homework on matter and the textbook doesn't elaborate much on that.

Answer 1

Both have optical properties which enable them to be transparent.
Both are good insulators of heat and electricity (i.e. they are poor conductors)
Both are relatively inexpensive to produce.
Both are easily shaped and moulded.
Both are non-toxic making them suitable for use with foodstuffs.
Both are fairly inert so they don't react with anything they are in contact with (i.e. they don't rust or dissolve away)

Answer 2

Both can be molded into useful shapes like bottles and jars, and both are waterproof. Therefore, they can do many of the same jobs.