Complete the space tourist's postcard back to Earth. Drag the correct words into the gaps.

The zero-gravity pool, which is located on the top deck, is incredibly fun to float around in. The alien staff members who run the gift shop are very friendly, even if they have six tentacles each. I bought a lot of souvenirs because everything that they sell is made of genuine moon rock!

The zero-gravity pool, which is located on the top deck, is incredibly fun to float around in.

Because this is a non-defining relative clause (set off by commas) describing a thing, we must use which. Where would require a subject to follow it (e.g., where I swim).

The alien staff members who run the gift shop are very friendly...

We use who for people (or personified beings like intelligent aliens) when they are the subject of the relative clause.

...everything that they sell is made of genuine moon rock!

After indefinite pronouns like everything, anything, nothing, or all, we strongly prefer that over which. What cannot be used as a relative pronoun following a noun or indefinite pronoun.