Imagine standing on a very tall hill in middle of a thick forest. Your hill is tall enough to take you above the trees; when you look out over the trees, you can see for miles around you. Ahead you can see another hill sticking above the trees; this is your goal.
You want to reach that hill.
A question arises; you are asked, “How long will it take to reach yon hill? What will you need along the way?”