Teething, and more teething, lol. Lots and Lots of chew toys, teach that it's not ok to chew on people. Watch carefully around small children. They mean no harm, but they are teething babies too and it hurts like crazy when they bite you or a kid. They will be teething for a while and at their hardest and losing teeth between 4 and 9 months. If they teeth on you, holler OWW and hold their mouth shut for a second. He will learn it hurts you. Heavy duty Rubber toys are the best like Kong and Nyla Bones. They cost but will last.