What stops perpetual machines

The first law of thermodynamics

Step-by-step explanation:

A perpetual motion machine can supposedly do work forever without an external energy source.

However, if it starts with a finite supply of energy, it will not operate indefinitely.

The first law of thermodynamics states that energy cannot be destroyed or created, but it can be converted from one form to another.

The machine's stored energy will be used up by being converted into the work that it does, so the device will eventually run down and stop.