Here's why:

* Sound needs a medium: Sound waves are vibrations that travel through a medium, like air, water, or solids. These vibrations cause molecules to bump into each other, transferring energy.

* Vacuum is empty: A vacuum is a space devoid of matter, meaning there are no molecules for the sound waves to vibrate. Without a medium, there's nothing for the vibrations to travel through.

Therefore, the speed of sound in a vacuum is zero.