discovered by Cosmo & Kuwaga
It is possible to write Lon-Lon Milk (half) to the B button.
The drinking code for OoT will check if what you're drinking is Lon-Lon Milk, because it is the only item that will not become an empty bottle after drinking it.
Full Milk has the value of 26 (decimal). Because the B button uses c-right as the pointer, it will look at the appropriate address, and if it is 26, drinking using the B button will result in half-milk. This writes 31 onto the memory address that had 26 previously.