What do his parents do at work all day and why do they come home to tired to play and read stories?

Both Mum and Dad work in offices, but it's hard for a very little boy to understand what they DO all day, and why they're SO tired when they get home and can only play for a little while. With the help of his grandmother, who takes care of him, our young narrator tries to experience their workaday world, day after day, at home in a pretend office...and he gets really, really tired, too...