Hello, hips, legs and feet!
Notice how your baby uses their hands when playing on their back. At first, they probably focused on exploring their head, chest and tummy areas using closed hands or big arm movements. As they have grown, they will move to touching more areas of their body, such as their hips, legs and feet.
Hands on, hands out
Just as they are getting to know more of themselves as their body grows, they will change how they use their hands as they develop new skills. Look forward to seeing them using their arms to reach further, their hands to grasp, and their fingers to point, tickle and squeeze.
Reference:
[1] Thomas, B. L., Karl, J. M., & Whishaw, I. Q. (2015). ‘Independent development of the Reach and the Grasp in spontaneous self-touching by human infants in the first 6 months.’ Frontiers in psychology, 5, 1526. Available online: https://doi.org/10.3389/fpsyg.2014.01526