It's a cup with transparent portions. Depending on the contents, Sans's eye and bones will change colors.
... if you use any color other than blue, he will turn into an AU. Be careful and drink (blue raspberry lemonade) responsibly.
This official UNDERTALE drinking glass was designed by Ade; it holds about 11.67 fl oz (345 ml). Sans's eye color is determined by the color of the liquid you're drinking.
