It's hard to believe that water has a lot of empty space. Try this next time you're ready to make lemonade. I suggest filling the glass halfway and marking the water level with a sharpie and noting if the water level changes when sugar is mixed in (filling the cup to the brim will surely result in spillage not because sugar is added but because of splashing from mixing). Keep track of how much sugar need to be added to see a change in the water level. Try the same activity with salt to see if you get different results. Then use the salty water to cook pasta for dinner.