A reflection always reproduces objects as a complete mirror image, rather than just its individual parts or individual parts in a completely different orientation. It’s all or nothing, the mirror can’t reflect just a little. This illustrates a fundamental symmetry principle in nature. For decades, physics assumed that the laws of nature in our world and in the mirror world would be identical, that parity would be preserved. Then in 1956, in the realm of elementary particles, or more precisely in the realm of the weak interaction, researchers discovered a violation of this principle. Parity violation has been a subject of scientific research ever since.