How full is your cup?

Today at snack time two boys starting comparing their cups of juice/water. They discussed who's cup has more or less in it. They put the cups close together, bent down to the level of the cup and compared the levels of the liquid. One of the boys took a sip put the cup back down to measure it again. They talked about how the levels go down when you take a sip out of the cup, then the boys both took a big sip from their cups put them back down to see who had more? They measured the liquid using their fingers, moving their finger from one cup to the other.