"The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings."
"Energy can neither be created nor destroyed, only changed in form."
— Law of Conservation of Energy
"To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions."
— Newton's Third Law of Motion
"Alchemy: the science of understanding, deconstructing, and reconstructing matter. However, it is not an all-powerful art; it is impossible to create something out of nothing. If one wishes to obtain something, something of equal value must be given. This is the Law of Equivalent Exchange, the basis of all alchemy. In accordance with this law, there is a taboo among alchemists: human transmutation is strictly forbidden - for what could equal the value of a human soul...?"
Ed: What I seek was just ahead... the truth about human transmutation! Please, show it to me again!
Truth: I can't. I can only show you this much for the toll you've paid.
Truth: Yes, toll. [Ed's leg disappears] It's an equivalent exchange, right? Alchemist?
"Oh, you could do it all by magic, you certainly could. You could wave a wand and get twinkly stars and a fresh-baked loaf. You could make fish jump out of the sea already cooked. And then, somewhere, somehow, magic would present its bill, which was always more than you could afford."
"There ain't no such thing as a free lunch."
"All magic comes with a price!"
— Rumpelstiltzkin, Once Upon a Time
And so, this is what the Pearl Satraps do. On one hand, they please themselves above all others. They go out in the world and what they see, they want, and what they want, they purchase. It's a mad game to serve their vanity, but what does one have if one is without the joy of conceit? One Satrap sees a silver locket around a woman's neck, and that locket holds the picture of a dearly departed husband. Oh, but the Satrap wants it, and the woman assures him she'll never give it up. But she will. Maybe money will do the trick (people love money more than they care to admit, the Satraps say). If not that, then something else. The return of a lost child? A threat against her dear friend? Revenge against a cruel boss? She'll give the locket up when the Satrap sounds the right price. And he'll take it only when she's conceded to that price; no Satrap will accept something without first paying the proper cost. Nor will he ever steal, for thievery is anathema to this noble order. Nothing is free. To act as if it is means a wilful refutation of those principles the Satrapy holds dear.
— Changeling: The Lost, writeup for the Satrapy Of Pearls group