The other day I watched my friend's baby. Except her baby is as much of a baby as my baby, meaning not so much. She is basically the same age as the Boy. When she realized that I was taking care of her, not her mom, and that we were at my house, not hers, she got very sad. So she cried.
The boy was amazing, and let me hold her without getting jealous. We read books, we played with toys, we went outside, nothing made her forget her sorrow. She would stop crying for a moment, then remember how bad things were, then start up again. She also would not let me put her down. Luckily she only really whine-cried, not full out screaming.
The Boy was confused by this constant sorrow, and crawled over to her, and gave her a hug. That made her even less happy, but it thrilled me to watch my little boy already recognize sadness, and know how we take care of it.
(Eventually I took them both downstairs and turned on the TV. Moments later she fell asleep on my lap, in what I refer to as 'protest sleep.')
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Although the Boy acts in similar ways. Every so often I find someone to watch him so I don't have to wake him to pick up one of his sisters at school. He inevitably wakes up anyway while I'm gone, and then will not let go of whomever picks him up, and snuggles with them until I get home. Then he tells me the sad story of his sleep ending and someone else being there.