Just because a solid dissolves in a liquid doesn’t mean the solid no longer occupies space. Fill a glass to its brim with the warm water, and then carefully pour all the water into the larger container. Add a couple tablespoons of sugar to the empty glass. Return half of the warm water to the glass and stir to dissolve all the sugar. Return the remaining water, and as you get close to the top, ask a friend to predict whether the water level will be lower than, about the same as, or higher than before. If your friend doesn’t understand the result, ask him or her what would happen if you had added the sugar to the glass when the glass was full of water.