Photo: Tim Dustrude/Dustrude Photography
I would've answered "fleshy deliciousness" (sorry, vegetarians!). That, or magic. But the folks at the USDA spoiled the fun by explaining it with science.
From Taylor Orci's post over at The Atlantic:
According to the USDA, "When light hits a slice of meat, it splits into colors like a rainbow." This is something called a "diffraction grating," essentially what happens when light waves bend or spread around a surface and create a pattern. It's the same thing that happens to make rainbows on the surface of a DVD. It's understandable that folks mistake diffracted light as a sign of spoilage, especially since the main color created by meat diffraction gratings is green. There is a reason why in Dr. Seuss's Green Eggs and Ham, the central conflict of the protagonist is his strong apprehension against eating green meat.