Imagine two astronauts working on the outside of a space station. one yells instructions to the other astronaut. why can't they hear each other?

There is an empty space, or sounds must travel through matter to make a sound

No you cannot hear any sounds in near- empty regions of space. Sound travels through the vibration of atoms and molecules in a medium (such as air or water). In space, where there is no air, sound has no way to travel

During early infancy, babies are extremely egocentric. They have no concept that the world exists separate from their point of view and experience. To understand that objects continue to exist even when they are unseen, infants must first develop a mental representation of the object.

hope this helps. Have a great night:)