When you have already been used to watching your baby always lying on his stomach or in flat position, you might just be amazed by the time you see them in a sitting a position. Plus that happens now without any assistance. This is actually the best time to strengthen their core muscles every time they practice this milestone. Try rolling a ball toward to further let him lean over and reach for it. Take note your babies are not robots that could manage to accomplish this to perfection, sometimes they topple of over every now and then but that what teach teaches them on how to really master it.
Along with this is how they reach or hold for small objects. They have gotten over with the palm grasp already and gotten past of the difficulty in reach for the smaller ones. Now even the smallest cereals can now be easily grasped with their index finger and thumb. But this means extra careful especially to objects that are choking hazard.
At the language aspect, it is now the perfect time to talk with them and teach some language as this period gives them increased attention to speech. How animated your stories can be is how strongly they pay attention to it especially with the additional facial expressions.