Yes, we kind of promised to never mention that dress again, but it serves as a way into the latest optical illusion "going viral" (as the kids say).
The premise is that these two images are actually the same.
But whereas the reason people saw the dress as either black and blue or white and gold was due to the way we perceive colours, the explanation behind the nails and lips image is actually much simpler: it's a Photoshop trick.
The image works perfectly on Twitter due to the way in which pictures have different backgrounds depending on whether you view them within your timeline (white) or when the image is viewed separately (black). The effect also works in different browsers.
The nails and lips are actually virtually transparent, with some red pixels, meaning the colour of the background determines what you see.
This is what the picture looks like in Photoshop, for example.