When puppies start teething they start chewing on thing and you too. Isally this happen at 3 months old
Puppies start developing teeth around 4 weeks of age. This is what also incourages the mother to start weening the pups, because those baby teeth are SHARP. Make sure mommy dog is eating good puppy food while she is still nursing puppies and make sure there is food and water available for mother dog AND puppies at all times. When pups are ready, you will see them "gumming" at the dry food and they will eventually start eating more dry food and nursing less from the mother. Some people say 6 weeks for weened puppies, but IMO I'd let them nurse from the mother until 8 weeks. By then they should be on straight dry food and water and can go to new homes.