Summarize this article in one sentence.
Puppies up to two weeks old are tiny in size. They haven't opened their eyes yet. The puppies do not move around very much and stay close to their mother. They mainly only root, feed from the mother, and sleep. Puppies open their eyes when they are around two to three weeks old. They start walking and adventuring out of the whelping box during this time. They appear to become more aware of their surroundings and they begin to explore on their own. This is characteristic of puppies up to around five to six weeks old. Puppies generally get their food from their mother until between five to seven weeks old. Puppies who are transitioning to eating solid foods and who are becoming independent of their mother are probably around six to eight weeks old. As the puppy grows, they start becoming more active. By eight weeks old, the puppy is acting how people typically expect a puppy to act. Because their teeth are coming in, they begin chewing on things. They also begin playing in an energetic manner and exploring everything around them. They may also start acting restless or whining due to their teeth. They will also spend a lot of time sleeping in between playing and chewing. Puppies have a growth spurt between eight and sixteen weeks of age, and then they continue visibly growing over the next several months. As they age, puppies will lose their puppy coats and gain their adult coat. Puppy coats are very soft, while adult coats are denser and thicker. Puppies who are between six to twelve months old will start testing their boundaries. They get an increase in energy and are distracted easily by everything. They may start chewing more things around the house, and they love to explore and may wander off. At this age, the puppy may ignore you and forget behaviors they previously learned. Most puppies will have reached their full size by the time they are between eight and twelve months old. Larger breeds will continue growing for up to two years. A dog under two years is still considered an adolescent. They have some puppy-like qualities, like chewing and energy, but they may have calmed some. Unspayed female dogs typically have their first heat at 6-24 months old. You can tell that your dog is in heat if she's nervous, very alert, and gets distracted easily. You also might notice that her vulva is swollen, or producing blood-tinged or straw-colored vaginal discharge. A male puppy's testicles should descend into its scrotum within the first 7 weeks after birth. You'll be able to tell because the testicles will be much more noticeable. If you are unable to figure out how old the puppy is based on the teeth or developmental stage, you can take them to the vet. The vet will be able to look at the size and breed, along with the teeth, and give you a fairly accurate estimated age. Whenever you first get a puppy, you should take them to the vet to get a checkup. The vet can tell you their age during your first visit.
Look for small puppies with closed eyes. Notice if the puppy is just starting to walk. Check to see if your puppy is eating solid food. Watch for increased chewing and playing. Monitor the puppy's growth. Watch for a change in their coat. Watch for behavioral problems. Determine if the puppy has slowed their growth and development. Observe whether a female puppy is in heat. Check a male puppy's genitals to get an idea of his age. Take the puppy to the vet.