Imagine that you have a bacterium that reproduces every minute, by splitting in half and doubling its numbers. You put one bacterium into a bottle of food at 8:00 AM, and let it grow. You come back at noon, and notice that, at the stroke of noon, the bacteria are just eating the last of the food and exactly filling the bottle with bacteria. They have turned a whole bottle of food into a bottle full of bacteria. The question is: "When was the bottle exactly one-quarter full of bacteria?"

If you try to calculate the answer going forwards in time from one bacterium, it is very difficult to solve.
But if you work backwards in time, the answer is pathetically easy:

* At noon, the bottle was exactly full.
* At one minute before noon, the bottle was half full.
* At two minutes before noon, the bottle was one quarter full.

You can continue that sequence backwards a few more times, and find that at seven minutes before noon, the bottle was only 1/128 full of bacteria -- less than one percent full. If they could have, the bacteria might have looked around and said to themselves, "We have miles and miles of empty space and tons of food left. We can reproduce forever."

Little did they realize that they were only seven minutes from the end.