At 12 weeks old, babies start to show more controlled movements and increased muscle strength. They can lift their head briefly when lying on their stomachs and may even attempt to roll over. They also begin to bring their hands together and may reach for objects within their line of sight. These movements are important for their physical development as they help strengthen muscles, improve coordination, and enhance motor skills. It is crucial to provide a safe environment for them to explore these new abilities under supervision.