it has always seemed to me that one of the main things that would be fun about having two kids is getting to do experiments on them

Robert Frank, Falling Behind, page 54:

I came to this view in part because of an experiment I did years ago with my two oldest sons when they were five and seven years old. The experiment took three days. On day one, I poured each of them a full glass of orange juice. On day two, I poured each only half a glass. Then, on day three, I poured David (then age seven) seven-eighths of a glass and Jason (then age five) only three-quarters of a glass. (I am not sure that a human subjects committee would approve this experiment today.)

You can guess what happened. On the first two days, each drank his juice without comment. In particular, neither asked on day two why he'd gotten only half as much as the day before. But things played out differently on day three. Jason looked first at his own glass, then over at his brother's, then back at his own, his face registering growing signs of distress. It was obvious that he was struggling not to react. But finally he blurted out, "That's not fair; he always gets more than me."