What is a relative clause?

What could it be?

The white liquid WHICH/THAT is produced by cows, goats, and sheep and used by humans as a drink or for making butter, cheese, etc

What could it be?

a large country in the southwest Pacific Ocean where kangaroos live

What could it be?

Famous Spanish painter WHOSE mustache was very peculiar

What could it be?

American national holiday on the fourth Thursday in November WHEN families get together for a big meal and express their appreciation for life, health, etc.

What happened?

This was WHY two split European countries became one in November of 1989