Chewing on hay serves multiple important purposes for guinea pigs. Firstly, it helps them wear down their constantly growing teeth. Secondly, it aids in their digestion. It is crucial to provide them with a constant supply of hay, available 24/7. If you choose to offer a pile of hay, make sure to replace it daily to ensure freshness. It is not recommended to use hay racks, as guinea pigs can potentially get their legs stuck in them, resulting in broken limbs and the need for surgical intervention. Instead, I would suggest feeding hay from a hay bag or simply providing a large pile for them to access easily.