Rai stones are round with a hole in the middle. The smallest are about the size of a dinner plate, but the biggest are over 3 metres across. That is taller than most ceilings in your school. Each stone is so heavy that many of them have never moved at all - they just sit in the same place in the village for hundreds of years.
Here is the amazing part: even when a stone does not move, its ownership can change. If someone does a big favour for another family, gives them food in a hard time, or trades something important, everyone in the village agrees that the stone now belongs to the other family. The stone stays where it is, but its owner is different. The whole village is the bank.
One rai stone famously fell off a canoe into the ocean. It was never recovered - it sank to the bottom of the sea. But people on Yap still counted it as real money! Because everyone agreed it existed and knew who owned it, it kept its value even underwater.
The most valuable stones are not necessarily the biggest - they are the ones with the best story. A stone brought back from a very dangerous voyage is worth more than an easy one. The adventure and effort are part of what makes it valuable.