The example I've come across was of musical chairs. If there are 3 seats and 10 people, no matter what, some people will be out of luck.
I find it strange reading all the "get an education" or "don't have babies out of wedlock" attacks. If everyone in the country got PhDs, we still have the same level of poverty. If everyone married and then had babies, we'd still have the same level of poverty.
Lets say every american went to medical school and become doctors. You know what we'd have? A lot of doctors in poverty. If everyone became a software developer like me, we'd have hordes of poor developers.
The dominant economic system ( quasi mercantilistic capitalism with some social protections ) today pretty much guarantees poverty for a portion of the population. It's structurally systematic. The system is designed for income inequality and no matter what, we will have few extraordinary wealthy and lots of people in poverty. This is the dominant trend with a few blips ( the burgeoning of the middle class post ww2 US, but that was an anomally ).
If everyone had PhDs then those PhDs would be able to produce many more goods and services than exist now.
If everyone was doctors, then healthcare would be plentiful, and nobody would suffer from not having access to healthcare.
If the world was full of software engineers, then we would have a plentiful amount of software.
This is easier to consider by thinking about the opposite situation.
Imagine if we got rid of all of the doctors, software engineers, and farmers? What would happen?
We would quickly lose access to all of our healthcare, new software and then all of our food.