Chiral Centers
In organic chemistry, a chiral center is a carbon atom with four unique bonds attached to it, i.e., bonds that connect to different atoms or chains of atoms. They are special, because if you take the mirror image of a molecule with a chiral center, you cannot rotate and translate it to align with the original molecule. It's a new molecule, distinguishable from the original! This is important in biology, where, e.g., our bodies may process one version of a molecule well, but not its mirror image. In some cases, we can process both with different results: one version of the molecule carvone smells like caraway while its mirror image smells like spearmint.
Consider the pairs of molecules below, with the original molecule and a reflected (mirrored) version of it in each row. The color of the attached atom (which could represent a chain of atoms) tells you how many of the four bonds from the central carbon atom are unique. Try to rotate the reflected molecule so that it looks just like the original. Can you do this successfully for all pairs? Hint: no! (Row 3 is both trickier intellectually, and trickier to manipulate due to the funky rotation interface that was needed and provided with three.js -- just keep trying.)