My Colorful Castanets work by clanging each castanet together. Hold one castanet in each hand. Put your thumb on the bottom and your index finger on the top. Then push together.
The reason the castanets work is because of the metal cookie cutters hitting each other, which makes a vibration. If they were plastic, they would not make the same noise.
The castanets are a percussion instrument because they work by banging two things together - the metal cookie cutters.