S1 spends a day in my office

Due to parent-teacher conferences, S1 has the day off from school today.  But neither the wife nor I can take the time off.  So, S1 did what he does in such situations -- he came to work with me.  Eight hours is a long time for a 7-year old to occupy himself.  But he manages.

First, he made a "drawing book".  He drew a bunch of pictures on paper, stapled them together and plans to take it home to give to S2 so that she can color them.

Next, he did a 100-piece Spiderman jigsaw puzzle. This occupied him for more than an hour.

Then, he commandeered a couple of packing boxes that have been lying around in my office for several months.  I was wondering what he was doing with them. It turns out to be a train.  He is sitting in the first car, reading a book. All his other stuff is in the second car.

So, he's managed to occupy himself for 6 hours now.  I can't imagine S2 being so patient and self-directed.  Difference between first and second children?