system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does carrying 40 pounds of water feel heavier than carrying a 40 pound child?
gpt
Because the kid wants to be comfortable, and the water doesn't.  

Deadweight, like the water, is harder to carry because it doesn't do anything like, say, maintain it's own center of gravity---or, in the case of an older kid, actually hold on to you, thus distributing the weight across more of your muscles.  

As a result, you need to use your own power to do all the little things necessary to keep the weight oriented they way you want it to be oriented.  That makes it seem heavier.  The water might slosh around or get slightly off center from how you are carrying it some other way, and you then have to consciously or sub-consciously correct it.  The kid, on the other hand, probably wants to stay generally upright and generally in the same place/orientation as after you lifted them, taking that pressure off.*

*assuming not a toddler. Their weight is effectively quintupled from sheer madness.