Here's why:

* Condensation is the process where a gas changes into a liquid. It requires the gas molecules to come together and form clusters.

* Nucleation is the formation of these initial tiny clusters of liquid molecules within the gas. These clusters are called *nuclei*.

* Nuclei can form on various surfaces, called *condensation nuclei*, such as dust particles, pollen, or even tiny salt crystals in the atmosphere.

* Once a nucleus forms, more gas molecules can condense onto it, causing it to grow larger and eventually becoming visible as liquid droplets.

So, nucleation is the essential first step for condensation to occur. Without it, the gas molecules would remain in a gaseous state.