Dogs should not eat acorns because acorns contain tannins, which are toxic to dogs. Eating acorns can cause stomach upset, vomiting, diarrhea, abdominal pain, and in severe cases, liver or kidney damage. Additionally, the hard shells of acorns pose a choking hazard and risk of intestinal blockage. Moldy acorns can also produce harmful mycotoxins that may cause neurological symptoms. If a dog eats acorns, it is a medical emergency, and a veterinarian should be contacted immediately. Preventive measures include keeping dogs away from areas with acorns and teaching commands like "drop" or "leave it" to prevent ingestion.