Fish tank:it is plastic because it has water in and it is see through,also it wouldn't be a fish tank if it wasn't plastic. Window frame:it is plastic so no rain can get in and also it secures the heat so their is no gaps. Dog cage:it is plastic because if it was wood then it would rot or bitten through and metal would be to sharp. Light switch:because if it was wood it would give you splinters and metal would be to sharp. Bowl:so it keeps the water from leaking And it doesn't rot and burst.