The rubber duck has baffled wizards of all ages and skill since its discovery by one unnamed wizard in a quaint muggle store while he was on his travels. There are numerous theories concerning the item, two of the most popular theories include many wizards and witches speculating that it functions as some sort of musical instrument invented by a somewhat eccentric muggle in the late 1800s, others believe it acts as a collectors item and functions in the same way as collection chocolate frog cards (sans enchantment and chocolate, of course). 

There are other speculations however that border on absurd, such as the rubber duck functioning as a companion for when on is bathing (a theory popularised by a muggleologist two decades ago) - ideas such as this are far less popular but are still entertained by a minute number of wizards with its justification being that muggles are just peculiar things. Another peculiar reaction was that of the house elves, while there is universal acceptance that the rubber duck is purely a muggle contraption, a number of the house elf population are convinced that the items are petrified magical creatures that have somehow stumbled into the muggle world; however we have come to the agreement that this is neither plausible nor realistic.

While observing the item these past few days; one is able to theorise a number of reasons for the items existence. In truth, ideas they have become rather odd as open discussion with muggle-borns seem to indicate that it is indeed a bath time companion for (typically, but not limited to) younger muggle children.

In conclusion, the function of the rubber duck remains open to discussion for most wizarding families, while within muggle households it seems like just another item in their strange way of living. Although I am not convinced by the answer some muggle-born students offered, there is no way to argue with their explanations if one if not of their world. It is indeed a strange world that muggles live in.