Why do guinea pigs grind their teeth?

Sometimes they grind their teeth when they're uncomfortable. It can be a sign of pain or discomfort. But sometimes they can do it when they're happy too. If you're scratching their chin and they grind it, they can do more like a teeth chattering thing- a tooth purr. It sounds like they're purring, but really it's their teeth. Otherwise, when they grind their teeth, they can be uncomfortable too. So it depends on the context of what's happening and if it's normal for them to purr.