Science proves Louis C.K. is right!
The comedian Louis C.K. has a routine in which he talks about his daughter’s understanding of fairness. He begins, “My five-year-old, the other day, one of her toys broke, and she demanded that I break her sister's toy to make it fair.” This would make the sisters equal but the joke here is that something here doesn’t feel right: “And I did. I was like crying. And I look at her. She’s got this creepy smile on her face.”
Other intuitions about fairness are simpler. Imagine you have two toys and two children, and you give both toys to one child. If the other child is old enough to speak, she will object. She might say “That’s not fair!” and she’d be correct. An even split would maximize the overall happiness of the children — give each child one toy and they’re both happy; divide them unevenly, and the child who gets nothing is miserable, her sadness outweighing the extra pleasure of the child who gets two. But more to the point, it’s just wrong to establish an inequity when you don’t have to.