Sam is four months and a week old, still a thriving and happy baby. Today, he cycled his arms in an effort to make lift-off from his dad's knee as he watched his sister circle the kitchen.
Whatever she does, he wants to do. Walk, run, dance, talk, sing, eat real food, play with toys. Watching a second child learn is different from watching a first child learn, because the second child learns so much from watching the first. He tries to talk like a child, not an adult - hearing his sister gives him more range and more options for self-expression. (Some of them shrill. Some of the quite shouty.)
Sam is more interested in toys and in manipulating objects than Anna. It could be a gender thing - he shrieked with delight the first time he played with Anna's firetruck with its wheels, ladders, and lights - but Stephen thinks it is simply an imitation thing. He sees Anna playing with toys and using objects, and he wants to as well.
Sam can flip from his belly to his back now, much to his surprise when he manages it, but he doesn't get to practise as much as Anna did at his age, mostly because Anna gives him no space at all to roll over.
All the same, I can see that these treasured and peaceful days with a baby who stays where you put him down are coming to an end, and quickly. Already, Sam kicks himself in circles and edges himself to edges. He has a body built for crawling and can already almost get his knees under him. His bum reaches an alarming height and will soon enough propel him forward and then there will be no stopping him. Those few little tasks that are now possible to accomplish with a just-set-down-for-a-moment-on-the-floor baby will soon be gone.
When he can catch up with his sister under his own steam, she will be the one who needs her parents' help to get breaks from a space-invading sibling.