Time management instructors often use a jar with a wide mouth as an illustration.
First, they fill up the jar with large rocks and say, "Is it full?" Most people say, "Yes."
Then they add small pebbles. As they shake the jar, the gravel fills up the spaces between the large rocks. They ask again, "Is it full?" "Yes," is the expected answer.
Then they add sand, again shaking the jar so that the particles find their place between the crevices.
Finally, water is added.
"Now, is the jar filled?" the expert asks. "Yes, it is completely filled," comes the reply.
Then the time management instructor makes his real point: "If you don't put the big rocks in first, you'll never get them in." His point is, do the most important things on your agenda first or you'll never get them done.