A Guinea pig's diet consists of three key components: high-quality commercial pellets explicitly formulated for Guinea pigs, fresh hay, and fresh vegetables. The pellets provide essential nutrients, including Vitamin C, which Guinea pigs cannot produce independently and must receive through their diet. Fresh hay, such as timothy hay, is crucial for dental health and digestion. Vegetables, especially leafy greens like romaine lettuce and bell peppers, offer additional Vitamin C and variety. Occasional small servings of fruits can be given as treats. Fresh, clean water should always be available.