Actually I had a magic set long ago with this trick in it. The cups have a mirror inside of the down the center of the cup except for the first one. the mirror makes it look like the cup is whole but depending on where the mirror is placed you can fill one side of the cup with less liquid. If you look carefully especially at the last cup you can actually see the top f this piece of miror near the rim of the cup. This is also why the cups have that diamond pattern on them, to obscure people from seeing a clear reflection.