“There ain't no such thing as a free lunch”

If someone is getting paid, someone is paying.

This is an adage that toes the line between a deepity (something that sounds profound but is entirely trivial) and genuinely profound truth. In many cases it's worth remembering TANSTAAFL because agents under capitalism will try to advertise something as "free" when it has hidden costs (either financial, or by using your data/input for profit). On the other hand, outside capitalism it's often a trivial retelling of thermodynamics: there's no free energy, everything has to come from somewhere.

Interestingly, although TANSTAAFL can be summarized as "everything is a zero-sum game", the core tenet of modern economy is that the economy is not a zero-sum game and eternal growth is possible. So as much as TANSTAAFL has been characterized as "libertarian", it's also a refutation of libertarian narratives like "trickle-down economics" and perpetual economic growth.

In practice there is though. If something is going to happen regardless whether you take advantage of it or not and you manage to benefit from it, then it basically was a free lunch for you.

One example of this is hydro power. Once you build a water mill on a river you don't have to do any work to keep it going. The sun heats up the water downstream from the waterwheel and it falls down as rain upstream. The river keeps flowing whether you take advantage of it or not. Eventually the sun will stop shining, but in practice that's so far away that it doesn't matter.