I hope this goes chiral.

So, a little background...

There are certain molecules that exist in two mirror forms of each other. They have the same four functional groups around a single atom but arranged in such a way that they are mirror images of each other but cannot be rotated in such a way so as to superimpose one molecule onto the other.

These molecules are called chiral. They show chirality.

This is the opposite of achiral structures that are identical to their mirror image but can be rotated in such a way so as to superimpose one molecule on the other. A sphere, for example is achiral. It's identical to its mirror image, but the sphere can be rotated so that it matches its mirror image, too.

Samuel L Jackson is chiral, however, because he is identical to his mirror image, but there is no way to rotate him so that he is identical to his mirror image.

That's the first part.

The second bit of background is that the two forms of chiral molecules in chemistry are referred to as being levorotary and dextrorotary because they each rotate the polarization plane of light either left (levo-) or right (dextro-). To shorthand this, the molecules are labeled as L- or D- such as L-alanine and D-alanine. 

So, after all that, this is funny because it's Samuel L. Jackson would be an enantiomer (the chiral version) of Samuel D. Jackson.

Weirdly this chirality has been hypothesized to present a problem to life from 'our world' as the opposite chiral version of molecules such as protein from a 'mirror world' would likely not be useful in cellular processes if brought into our world. Just this past year I read an admittedly not great run of the Fantastic Four that used this concept as a problem that the First Family of Marvel had to solve.