Inheritance

My dad told me this one when I was young, it's my favorite.

A father lay on his death bed. He had three sons. Trying to decide how to best bequeath his farm, he gave his sons a test. He gave each of his sons $1 and said to them, "I will give the entirety of my property to whoever can use this $1 to buy something that will fill up the largest portion of this room."

The sons all headed for the market.

The oldest son saw that there was exactly one hay bale left for sale. He used his $1 to purchase the bale and carried it back home. In the room, he untied the bale and scattered the hay about. It took up a small quarter of the room.

The middle son saw that there was a giant barrel for sale. He purchased the barrel, took it home, and filled it with water. He returned to the room, and dumped the water on the ground. It drenched the entire floor.

The two sons were confident in their purchases, knowing that their youngest and smallest brother would not be able to carry anything heavier than a hay bale or a barrel.

The youngest son finally returned home with his object. This object filled the entire room. His father immediately bequeathed his property to the youngest son.

What did the youngest son buy?