To set the scene, the party had just gone up a flight of stairs in a round tower, 30 feet in diameter, and came up into a half-moon shaped tower floor chamber that took up half the floor. They opened the door in the center of the dividing wall to see the other half-moon shaped chamber on the other side and a flock of harpies. The new MU (the party are all 5-8th level) announces he's casting fireball into the chamber with the harpies, to which the rest of the players yell in unison, "No!" realizing that his fireball blast radius is greater than the diameter of the entire tower floor.
"Too late." I say with a grin. "Everyone make a saving throw."
To the new MU player's credit, he said he never, ever understood radius/diameter/math. "Well, this is a practical problem for you then, and the EXPERIENCE you gain, should you and the rest of the players survive, will serve you well." I replied.

