Final answer:
Polyethylene bottles are made of HDPE with a non-flexible structure, while beach balls are made of LDPE with a flexible structure.
Step-by-step explanation:
Polyethylene is a type of polymer made up of repeating units called monomers. The properties of polyethylene can vary depending on the length of the polymer chains and how efficiently they pack together. Hard, strong polyethylene objects like bottles are made of high-density polyethylene (HDPE) with fewer branches, resulting in a non-flexible structure. In contrast, beach balls need to be flexible, so they are made of low-density polyethylene (LDPE) with many branches, creating a more flexible structure.