...is that you don't talk about thermodynamics.

Okay, seriously now. The first law of thermodynamics is that energy and matter (which are essentially the same) can neither be created or destroyed; they only change form.

For instance: I eat a hamburger. My stomach digests the hamburger and breaks it down into simpler chemicals. I metabolize some of these chemicals and derive chemical energy from them. I use some of that energy to turn the crank on my wind-up radio. The mechanisms inside the radio convert that motion into electricity, which, combined with a radio signal from outside, causes the speaker in the radio to vibrate in a certain pattern. When that pattern interacts with my eardrum, I perceive it as sound. Even this involves a transfer of energy; when my eardrum vibrates, it triggers electrochemical signals in my brain.

Though the hamburger has been through several changes, the energy/matter in it has never disappeared. If we were so inclined, we could also trace the energy backwards from the burger: the cow who provided its flesh for the hamburger got the energy for it from grass, which in turn photosynthesized its energy from sunlight. And so and so forth, right back to the Big Bang.