Gases are different, though. Gases neither have a fixed shape, nor a fixed volume. Gas particles move freely in whatever space is available to them.

So, even if Jerry mouse is in a different room, he can still smell the pizza. That is, initially, the pizza's aroma was contained in the pizza box. As the box was opened, the volume of the aroma increased to the volume of the room on the right, and later to the total volume of both rooms.