"Whenever you read a book or have a conversation, the experience causes physical changes in your brain. It's a little frightening to think that every time you walk away from an encounter, your brain has been altered, sometimes permanently."

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The total quantity of all the forces capable of work in the whole universe remains eternal and unchanged throughout all their changes. All change in nature amounts to this, that force can change its form and locality, without its quantity being changed. The universe possesses, once for all, a store of force which is not altered by any change of phenomena, can neither be increased nor diminished, and which maintains any change which takes place on it.

— Hermann von Helmholtz