Take a pinch of the spores of some rapidly-growing fungus and drop them into a gallon jug, one whose walls are coated with nutrient so the fungus can grow as fast as it likes. Suppose you do this one morning, and that exactly 4 days later you find that the fungus has filled the jar and is just beginning to overflow it. How full was the jug the morning of the 3rd day?

It was half full.