Rocket, it is really fairly simple....
You go to the pub and buy a jug of beer, but you only get a pot/middy (depends on where you come from), to poor it into.
Obviously, you are not going to be able to pour the contents of the jug into the pot/middy, until you have consumed some of what is in the pot/middy otherwise it will over flow and be wasted.........
The ATPL subjects are the 'Jug', and your brain, my friend, is only a middy.