My wife and I were cookin' a Cajun feast for the anniversaire de ma mere. While I handled the vittles, the lovely and talented Mrs. Boy made the drinks.
  She had made the tea strong and wanted to dilute it with 4 cups of water but the guests were at the door and the tea was still hot so she decided to dilute it with ice instead.
  She turned to me and said, "Fat, sweetie, how many ice cubes make a cup of water?"
  I confessed that I did not know as I had not measured the water when I made the cubes. To make matters worse I had not paid attention to how full I had made the trays so we couldn't just refill them and see how much they held.
  Things seemed desperate, as I'd die before I'd serve my Gumbo without sweet tea, but Mrs. Boy is no fool and she found a way. The tea was just right (though the cheese grits were a little burnt).

How did Mrs. B manage to ascertain the proper number of ice cubes to produce the 4 cups of water needed to dilute the tea? All she had to use was the ice cubes themselves, an ungraduated glass pitcher of unknown volume and the 4 cup graduated Pyrex measuring cup full of (too strong) tea.

First pour the concentrated tea into the pitcher.

Next, pour water (as cold as possible) into the measuring cup to level of either the 1 or 2 cups line.

Add ice until the water level reached an additional cup,  then carefully pour out the water leaving the correct amount of ice.

Add ice to the tea. The ice should have displaced the water one cup's worth!