Let us use a metaphor of pump priming to illustrate the complexity of the task.

Let us say your house is on fire, and you have to put it out. You are only three persons, there are two buckets, an open basin somewhat far away, and a long hose attached to a good efficient pump which has to be primed before you can pump lots of water on the flames. What would you do? Just to carry water from basin directly onto the flames leaves one person uninvolved (only two buckets) hence resources wasted. Use two people with buckets to prime the pump while the third one is pumping, and when the pump is primed two will pump and one will direct the hose? Then the fire might spread too far as it keeps burning while you prime, then even the pump will not extinguish the fire as it is too late. Have one person pump, another one carry water for pump priming, and the third one directly onto flames? Then it might be not enough water to prime the pump at all as one-two buckets are not enough, and it leaks while you bring more. Got the picture? The expected benefit is that after your pump is primed it is enough to have two people – one to pump and another to direct the hose, and the third can carry things out of the fire.

The same problem may be further aggravated if two people are called back to base as soon as the pump is primed. The remaining one can either pump or point the hose at flames, but cannot do both at the same time, and the fire spreads again, while he runs back and forth with the bucket. Then the base sends the two people back, and read above from the start. Hence the trick is to convince one more person to stay for a while after the pump is primed.